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Types
There are several categories of aircraft which qualify as ultralights in some countries:
Fixed-wing aircraft: traditional airplane-style designs.
Weight-shift control trike: use a hang glider-style wing, below which is suspended a three-wheeled carriage which carries the engine and aviators. These aircraft are controlled by pushing against a horizontal control bar in roughly the same way as a hang glider pilot flies.
Powered parachute: fuselage-mounted engines with parafoil wings, which are wheeled aircraft.
Powered paraglider: backpack engines with parafoil wings, which are foot-launched.
Powered hang glider: motorized foot-launched hang glider harness.
Autogyro: rotary wing with fuselage-mounted engine, a gyrocopter is different from a helicopter in that the rotating wing is not powered, the engine provides forward thrust and the airflow through the rotary blades causes them to autorotate or "spin up" thereby creating lift.
Helicopter: there are a number of single-seat and two-place helicopters which fall under the microlight categories in countries such as New Zealand. However, few helicopter designs fall within the more restrictive ultralight category defined in the United States of America.
Hot air balloon: there are numerous ultralight hot air balloons in the US, and several more have been built and flown in France and Australia in recent years. Some ultralight hot air balloons are hopper balloons, while others are regular hot air balloons that carry passengers in a basket.
Electric
Advancements in batteries, motors, and motor controllers has led to some practical production electric propulsion systems for some ultralight applications. In many ways, ultralights are a good application for electric power as some models are capable of flying with low power, which allows longer duration flights on battery power.
In 2007, the first pioneering company in this field, the Electric Aircraft Corporation, began offering engine kits to convert ultralight weight shift trikes to electric power. The 18 hp motor weighs and an efficiency of 90% is claimed by designer Randall Fishman. The battery consists of a lithium-polymer battery pack of 5.6kWh which provides 1.5 hours of flying in the trike application. The company claimed a flight recharge cost of 60 cents in 2007.
A significant obstacle to the adoption of electric propulsion for ultralights in the U.S. is the weight of the battery, which is considered part of the empty weight of the aircraft despite efforts to have it considered as fuel. As the specific energy of batteries improves, lighter batteries can be used. | Ultralight aviation | Wikipedia | 510 | 159298 | https://en.wikipedia.org/wiki/Ultralight%20aviation | Technology | Types of aircraft | null |
Urination is the release of urine from the bladder to the outside of the body. Urine is released through the urethra and exits the penis or vulva through the urinary meatus in placental mammals, but is released through the cloaca in other vertebrates. It is the urinary system's form of excretion. It is also known medically as micturition, voiding, uresis, or, rarely, emiction, and known colloquially by various names including peeing, weeing, pissing, and euphemistically number one. The process of urination is under voluntary control in healthy humans and other animals, but may occur as a reflex in infants, some elderly individuals, and those with neurological injury. It is normal for adult humans to urinate up to seven times during the day.
In some animals, in addition to expelling waste material, urination can mark territory or express submissiveness. Physiologically, urination involves coordination between the central, autonomic, and somatic nervous systems. Brain centres that regulate urination include the pontine micturition center, periaqueductal gray, and the cerebral cortex.
Anatomy and physiology
Anatomy of the bladder and outlet
The main organs involved in urination are the urinary bladder and the urethra. The smooth muscle of the bladder, known as the detrusor, is innervated by sympathetic nervous system fibers from the lumbar spinal cord and parasympathetic fibers from the sacral spinal cord. Fibers in the pelvic nerves constitute the main afferent limb of the voiding reflex; the parasympathetic fibers to the bladder that constitute the excitatory efferent limb also travel in these nerves. Part of the urethra is surrounded by the male or female external urethral sphincter, which is innervated by the somatic pudendal nerve originating in the cord, in an area termed Onuf's nucleus. | Urination | Wikipedia | 414 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
Smooth muscle bundles pass on either side of the urethra, and these fibers are sometimes called the internal urethral sphincter, although they do not encircle the urethra. Further along the urethra is a sphincter of skeletal muscle, the sphincter of the membranous urethra (external urethral sphincter). The bladder's epithelium is termed transitional epithelium which contains a superficial layer of dome-like cells and multiple layers of stratified cuboidal cells underneath when evacuated. When the bladder is fully distended the superficial cells become squamous (flat) and the stratification of the cuboidal cells is reduced in order to provide lateral stretching.
Physiology
The physiology of micturition and the physiologic basis of its disorders are subjects about which there is much confusion, especially at the supraspinal level. Micturition is fundamentally a spinobulbospinal reflex facilitated and inhibited by higher brain centers such as the pontine micturition center and, like defecation, subject to voluntary facilitation and inhibition. | Urination | Wikipedia | 240 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
In healthy individuals, the lower urinary tract has two discrete phases of activity: the storage (or guarding) phase, when urine is stored in the bladder; and the voiding phase, when urine is released through the urethra. The state of the reflex system is dependent on both a conscious signal from the brain and the firing rate of sensory fibers from the bladder and urethra. At low bladder volumes, afferent firing is low, resulting in excitation of the outlet (the sphincter and urethra), and relaxation of the bladder. At high bladder volumes, afferent firing increases, causing a conscious sensation of urinary urge. Individual ready to urinate consciously initiates voiding, causing the bladder to contract and the outlet to relax. Voiding continues until the bladder empties completely, at which point the bladder relaxes and the outlet contracts to re-initiate storage. The muscles controlling micturition are controlled by the autonomic and somatic nervous systems. During the storage phase, the internal urethral sphincter remains tense and the detrusor muscle relaxed by sympathetic stimulation. During micturition, parasympathetic stimulation causes the detrusor muscle to contract and the internal urethral sphincter to relax. The external urethral sphincter (sphincter urethrae) is under somatic control and is consciously relaxed during micturition.
In infants, voiding occurs involuntarily (as a reflex). The ability to voluntarily inhibit micturition develops by the age of two–three years, as control at higher levels of the central nervous system develops. In the adult, the volume of urine in the bladder that normally initiates a reflex contraction is about . | Urination | Wikipedia | 362 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
Storage phase
During storage, bladder pressure stays low, because of the bladder's highly compliant nature. A plot of bladder (intravesical) pressure against the depressant of fluid in the bladder (called a cystometrogram), will show a very slight rise as the bladder is filled. This phenomenon is a manifestation of the law of Laplace, which states that the pressure in a spherical viscus is equal to twice the wall tension divided by the radius. In the case of the bladder, the tension increases as the organ fills, but so does the radius. Therefore, the pressure increase is slight until the organ is relatively full. The bladder's smooth muscle has some inherent contractile activity; however, when its nerve supply is intact, stretch receptors in the bladder wall initiate a reflex contraction that has a lower threshold than the inherent contractile response of the muscle.
Action potentials carried by sensory neurons from stretch receptors in the urinary bladder wall travel to the sacral segments of the spinal cord through the pelvic nerves. Since bladder wall stretch is low during the storage phase, these afferent neurons fire at low frequencies. Low-frequency afferent signals cause relaxation of the bladder by inhibiting sacral parasympathetic preganglionic neurons and exciting lumbar sympathetic preganglionic neurons. Conversely, afferent input causes contraction of the sphincter through excitation of Onuf's nucleus, and contraction of the bladder neck and urethra through excitation of the sympathetic preganglionic neurons.
Diuresis (production of urine by the kidney) occurs constantly, and as the bladder becomes full, afferent firing increases, yet the micturition reflex can be voluntarily inhibited until it is appropriate to begin voiding.
Voiding phase
Voiding begins when a voluntary signal is sent from the brain to begin urination, and continues until the bladder is empty. | Urination | Wikipedia | 394 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
Bladder afferent signals ascend the spinal cord to the periaqueductal gray, where they project both to the pontine micturition center and to the cerebrum. At a certain level of afferent activity, the conscious urge to void or urination urgency, becomes difficult to ignore. Once the voluntary signal to begin voiding has been issued, neurons in the pontine micturition center fire maximally, causing excitation of sacral preganglionic neurons. The firing of these neurons causes the wall of the bladder to contract; as a result, a sudden, sharp rise in intravesical pressure occurs. The pontine micturition center also causes inhibition of Onuf's nucleus, resulting in relaxation of the external urinary sphincter. When the external urinary sphincter is relaxed urine is released from the urinary bladder when the pressure there is great enough to force urine to flow out of the urethra. The micturition reflex normally produces a series of contractions of the urinary bladder.
The flow of urine through the urethra has an overall excitatory role in micturition, which helps sustain voiding until the bladder is empty.
Many men, and some women, may sometimes briefly shiver after or during urination.
After urination, the female urethra empties partially by gravity, with assistance from muscles. Urine remaining in the male urethra is expelled by several contractions of the bulbospongiosus muscle, and, by some men, manual squeezing along the length of the penis to expel the rest of the urine.
For land mammals over 1 kilogram, the duration of urination does not vary with body mass, being dispersed around an average of 21 seconds (standard deviation 13 seconds), despite a 4 order of magnitude (1000×) difference in bladder volume. This is due to increased urethra length of large animals, which amplifies gravitational force (hence flow rate), and increased urethra width, which increases flow rate. For smaller mammals a different phenomenon occurs, where urine is discharged as droplets, and urination in smaller mammals, such as mice and rats, can occur in less than a second. The posited benefits of faster voiding are decreased risk of predation (while voiding) and decreased risk of urinary tract infection.
Voluntary control | Urination | Wikipedia | 492 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
The mechanism by which voluntary urination is initiated remains unsettled. One possibility is that the voluntary relaxation of the muscles of the pelvic floor causes a sufficient downward tug on the detrusor muscle to initiate its contraction. Another possibility is the excitation or disinhibition of neurons in the pontine micturition center, which causes concurrent contraction of the bladder and relaxation of the sphincter.
There is an inhibitory area for micturition in the midbrain. After transection of the brain stem just above the pons, the threshold is lowered and less bladder filling is required to trigger it, whereas after transection at the top of the midbrain, the threshold for the reflex is essentially normal. There is another facilitatory area in the posterior hypothalamus. In humans with lesions in the superior frontal gyrus, the desire to urinate is reduced and there is also difficulty in stopping micturition once it has commenced. However, stimulation experiments in animals indicate that other cortical areas also affect the process.
The bladder can be made to contract by voluntary facilitation of the spinal voiding reflex when it contains only a few milliliters of urine. Voluntary contraction of the abdominal muscles aids the expulsion of urine by increasing the pressure applied to the urinary bladder wall, but voiding can be initiated without straining even when the bladder is nearly empty.
Voiding can also be consciously interrupted once it has begun, through a contraction of the perineal muscles. The external sphincter can be contracted voluntarily, which will prevent urine from passing down the urethra.
Experience of urination
The need to urinate is experienced as an uncomfortable, full feeling. It is highly correlated with the fullness of the bladder. In many males the feeling of the need to urinate can be sensed at the base of the penis as well as the bladder, even though the neural activity associated with a full bladder comes from the bladder itself, and can be felt there as well. In females the need to urinate is felt in the lower abdomen region when the bladder is full. When the bladder becomes too full, the sphincter muscles will involuntarily relax, allowing urine to pass from the bladder. Release of urine is experienced as a lessening of the discomfort.
Disorders
Clinical conditions
Many clinical conditions can cause disturbances to normal urination, including: | Urination | Wikipedia | 491 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
Urinary incontinence, the inability to hold urine
Stress incontinence, incontinence as a result of external mechanical disturbances
Urge incontinence, incontinence that occurs as a result of the uncontrollable urge to urinate
Mixed incontinence, a combination of the two types of incontinence
Urinary retention, the inability to initiate urination
Overactive bladder, a strong urge to urinate, usually accompanied by detrusor overactivity
Interstitial cystitis, a condition characterized by urinary frequency, urgency, and pain
Prostatitis, an inflammation of the prostate gland that can cause urinary frequency, urgency, and pain
Benign prostatic hyperplasia, an enlargement of the prostate that can cause urinary frequency, urgency, retention, and the dribbling of urine
Urinary tract infection, which can cause urinary frequency and dysuria
Polyuria, abnormally large production of urine, associated with, in particular, diabetes mellitus (types 1 and 2), and diabetes insipidus
Oliguria, low urine output, usually due to a problem with the upper urinary tract
Anuria refers to absent or almost absent urine output.
Micturition syncope, a vasovagal response which may cause fainting.
Paruresis, the inability to urinate in the presence of others, such as in a public toilet.
Bladder sphincter dyssynergia, a discoordination between the bladder and external urethral sphincter as a result of brain or spinal cord injury
A drug that increases urination is called a diuretic, whereas antidiuretics decrease the production of urine by the kidneys.
Experimentally induced disorders | Urination | Wikipedia | 367 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
There are three major types of bladder dysfunction due to neural lesions: (1) the type due to interruption of the afferent nerves from the bladder; (2) the type due to interruption of both afferent and efferent nerves; and (3) the type due to interruption of facilitatory and inhibitory pathways descending from the brain. In all three types the bladder contracts, but the contractions are generally not sufficient to empty the viscus completely, and residual urine is left in the bladder. Paruresis, also known as shy bladder syndrome, is an example of a bladder interruption from the brain that often causes total interruption until the person has left a public area. These people (males) may have difficulty urinating in the presence of others and will consequently avoid using urinals without dividers or those directly adjacent to another person. Alternatively, they may opt for the privacy of a stall or simply avoid public toilets altogether.
Deafferentation
When the sacral dorsal roots are cut in experimental animals or interrupted by diseases of the dorsal roots such as tabes dorsalis in humans, all reflex contractions of the bladder are abolished. The bladder becomes distended, thin-walled, and hypotonic, but there are some contractions because of the intrinsic response of the smooth muscle to stretch.
Denervation
When the afferent and efferent nerves are both destroyed, as they may be by tumors of the cauda equina or filum terminale, the bladder is flaccid and distended for a while. Gradually, however, the muscle of the "decentralized bladder" becomes active, with many contraction waves that expel dribbles of urine out of the urethra. The bladder becomes shrunken and the bladder wall hypertrophied. The reason for the difference between the small, hypertrophic bladder seen in this condition and the distended, hypotonic bladder seen when only the afferent nerves are interrupted is not known. The hyperactive state in the former condition suggests the development of denervation hypersensitization even though the neurons interrupted are preganglionic rather than postganglionic. | Urination | Wikipedia | 448 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
Spinal cord injury
During spinal shock, the bladder is flaccid and unresponsive. It becomes overfilled, and urine dribbles through the sphincters (overflow incontinence). After spinal shock has passed, a spinally mediated voiding reflex ensues, although there is no voluntary control and no inhibition or facilitation from higher centers. Some paraplegic patients train themselves to initiate voiding by pinching or stroking their thighs, provoking a mild mass reflex. In some instances, the voiding reflex becomes hyperactive. Bladder capacity is reduced and the wall becomes hypertrophied. This type of bladder is sometimes called the spastic neurogenic bladder. The reflex hyperactivity is made worse, and may be caused, by infection in the bladder wall.
Techniques
Young children
A common technique used in many developing nations involves holding the child by the backs of the thighs, above the ground, facing outward, in order to urinate.
Fetal urination
The fetus urinates hourly and produces most of the amniotic fluid in the second and third trimester of pregnancy. The amniotic fluid is then recycled by fetal swallowing.
Urination after injury
Occasionally, if a male's penis is damaged or removed, or a female's genitals/urinary tract is damaged, other urination techniques must be used. Most often in such cases, doctors will reposition the urethra to a location where urination can still be accomplished, usually in a position that would promote urination only while seated/squatting, though a permanent urinary catheter may be used in rare cases.
Alternative urination tools
Sometimes urination is done in a container such as a bottle, urinal, bedpan, or chamber pot (also known as a gazunder). A container or wearable urine collection device may be used so that the urine can be examined for medical reasons or for a drug test, for a bedridden patient, when no toilet is available, or there is no other possibility to dispose of the urine immediately.
An alternative solution (for traveling, stakeouts, etc.) is a special disposable bag containing absorbent material that solidifies the urine within seconds, making it convenient and safe to store and dispose of later. | Urination | Wikipedia | 475 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
It is possible for both sexes to urinate into bottles in case of emergencies. The technique can help children to urinate discreetly inside cars and in other places without being seen by others. A female urination device can assist women and girls in urinating while standing or into a bottle.
In microgravity, excrement tends to float freely, so astronauts use a specially designed space toilet, which uses suction to collect and recycle urine; the space toilet also has a receptacle for defecation.
Social and cultural aspects
Art
A puer mingens is a figure in a work of art depicted as a prepubescent boy in the act of urinating, either actual or simulated. The puer mingens could represent anything from whimsy and boyish innocence to erotic symbols of virility and masculine bravado.
Toilet training
Babies have little socialized control over urination within traditions or families that do not practice elimination communication and instead use diapers. Toilet training is the process of learning to restrict urination to socially approved times and situations. Consequently, young children sometimes develop nocturnal enuresis.
Facilities
It is socially more accepted and more environmentally hygienic for those who are able, especially when indoors and in outdoor urban or suburban areas, to urinate in a toilet. Public toilets may have urinals, usually for males, although female urinals exist, designed to be used in various ways.
Urination without facilities
Acceptability of outdoor urination in a public place other than at a public urinal varies with the situation and with customs. Potential disadvantages include a dislike of the smell of urine, and exposure of genitals. It can be avoided or mitigated by going to a quiet place and/or facing a tree or wall if urinating standing up, or while squatting, hiding the back behind walls, bushes, or a tree.
Portable toilets (port-a-potties) are frequently placed in outdoor situations where no immediate facility is available. These need to be serviced (cleaned out) on a regular basis. Urination in a heavily wooded area is generally harmless, actually saves water, and may be condoned for males (and less commonly, females) in certain situations as long as common sense is used. Examples (depending on circumstances) include activities such as camping, hiking, delivery driving, cross country running, rural fishing, amateur baseball, golf, etc. | Urination | Wikipedia | 494 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
The more developed and crowded a place is, the more public urination tends to be objectionable. In the countryside, it is more acceptable than in a street in a town, where it may be a common transgression. Often this is done after the consumption of alcoholic beverages, which causes production of additional urine as well as a reduction of inhibitions. One proposed way to inhibit public urination due to drunkenness is the Urilift, which is disguised as a normal manhole by day but raises out of the ground at night to provide a public restroom for bar-goers.
In many places, public urination is punishable by fines, though attitudes vary widely by country. In general, females are less likely to urinate in public than males. Women and girls, unlike men and boys, are restricted in where they can urinate conveniently and discreetly.
The 5th-century BC historian Herodotus, writing on the culture of the ancient Persians and highlighting the differences with those of the Greeks, noted that to urinate in the presence of others was prohibited among Persians.
There was a popular belief in the UK, that it was legal for a man to urinate in public so long as it occurred on the rear wheel of his vehicle and he had his right hand on the vehicle, but this is not true. Public urination still remains more accepted by males in the UK, although British cultural tradition itself seems to find such practices objectionable.
In Islamic toilet etiquette, it is haram to urinate while facing the Qibla, or to turn one's back to it when urinating or relieving bowels, but modesty requirements for females make it impossible for girls to relieve themselves without facilities. When toilets are unavailable, females can relieve themselves in Laos, Russia and Mongolia in emergency, but it remains less accepted for females in India even when circumstances make this a highly desirable option.
Women generally need to urinate more frequently than men, but as opposed to the common misconception, it is not due to having smaller bladders. Resisting the urge to urinate because of lack of facilities can promote urinary tract infections which can lead to more serious infections and, in rare situations, can cause renal damage in women. Female urination devices are available to help women to urinate discreetly, as well to help them urinate while standing. | Urination | Wikipedia | 482 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
Sitting, standing, or squatting
Techniques and body postures while urinating vary across cultures. Different anatomical conditions in men and women may presume different postures, yet these are largely shaped by cultural norms, types of clothing, and the sanitary facilities available. While sitting toilets are the most common form in Western countries, squat toilets are common in Asia, Africa, and the Arab world. Urinals for men are widespread worldwide, although women's urinals are available in some countries, recently becoming more common in Western countries. With the spread of pants among women, a standing posture became impractical, but in some regions where women wear traditional skirts or robes, an upright posture is common.
Males
Cultures around the world differ regarding socially accepted voiding positions and preferences: in the Middle-East and Asia, the squatting position was more prevalent, while in the Western world the standing and sitting positions were more common. For practising Muslim men, the genital modesty of squatting is also associated with proper cleanliness requirements or awrah. In Western culture, the standing position is regarded as the more efficient option among healthy males. In restrooms without urinals, and sometimes at home, men may be urged to use the sitting position as to diminish spattering of urine.
Elderly males with prostate gland enlargement may benefit from sitting down to urinate, with the seated voiding position found superior as compared with standing in elderly males with benign prostate hyperplasia.
Females
In Western culture, females usually sit or squat for urination, depending on what type of toilet they use; a squat toilet is used for urination in a squatting position. Women averting contact with a toilet seat may employ a partial squatting position (or "hovering"), similar to using a female urinal. However, this may not completely void the bladder.
Females may also urinate while standing, and while clothed. It is common for women in various regions of Africa to use this position when they urinate, as do women in Laos. Herodotus described a similar custom in ancient Egypt. An alternative method for women voiding while standing is to use a female urination device to assist.
Talking about urination
In many societies and in many social classes, even mentioning the need to urinate is seen as a social transgression, despite it being a universal need. Many adults avoid stating that they need to urinate. | Urination | Wikipedia | 495 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
Many expressions exist, some euphemistic and some vulgar. For example, centuries ago the standard English word (both noun and verb, for the product and the activity) was "piss", but subsequently "pee", formerly associated with children, has become more common in general public speech. Since elimination of bodily wastes is, of necessity, a subject talked about with toddlers during toilet training, other expressions considered suitable for use by and with children exist, and some continue to be used by adults, e.g. "weeing", "doing/having a wee-wee", "to tinkle", "go potty", "go pee pee".
Other expressions include "squirting" and "taking a leak", and, predominantly by younger persons for outdoor female urination, "popping a squat", referring to the position many women adopt in such circumstances. National varieties of English show creativity. American English uses "to whiz". Australian English has coined "I am off to take a Chinese singing lesson", derived from the tinkling sound of urination against the China porcelain of a toilet bowl. British English uses "going to see my aunt", "going to see a man about a dog", "to piddle", "to splash (one's) boots", as well as "to have a slash", which originates from the Scottish term for a large splash of liquid. One of the most common, albeit old-fashioned, euphemisms in British English is "to spend a penny", a reference to coin-operated pay toilets, which used (pre-decimalisation) to charge that sum.
Use in language | Urination | Wikipedia | 348 | 159421 | https://en.wikipedia.org/wiki/Urination | Biology and health sciences | Basics | Biology |
Flight or flying is the motion of an object through an atmosphere, or through the vacuum of outer space, without contacting any planetary surface. This can be achieved by generating aerodynamic lift associated with gliding or propulsive thrust, aerostatically using buoyancy, or by ballistic movement.
Many things can fly, from animal aviators such as birds, bats and insects, to natural gliders/parachuters such as patagial animals, anemochorous seeds and ballistospores, to human inventions like aircraft (airplanes, helicopters, airships, balloons, etc.) and rockets which may propel spacecraft and spaceplanes.
The engineering aspects of flight are the purview of aerospace engineering which is subdivided into aeronautics, the study of vehicles that travel through the atmosphere, and astronautics, the study of vehicles that travel through space, and ballistics, the study of the flight of projectiles.
Types of flight
Buoyant flight
Humans have managed to construct lighter-than-air vehicles that raise off the ground and fly, due to their buoyancy in the air.
An aerostat is a system that remains aloft primarily through the use of buoyancy to give an aircraft the same overall density as air. Aerostats include free balloons, airships, and moored balloons. An aerostat's main structural component is its envelope, a lightweight skin that encloses a volume of lifting gas to provide buoyancy, to which other components are attached.
Aerostats are so named because they use "aerostatic" lift, a buoyant force that does not require lateral movement through the surrounding air mass to effect a lifting force. By contrast, aerodynes primarily use aerodynamic lift, which requires the lateral movement of at least some part of the aircraft through the surrounding air mass.
Aerodynamic flight
Unpowered flight versus powered flight
Some things that fly do not generate propulsive thrust through the air, for example, the flying squirrel. This is termed gliding. Some other things can exploit rising air to climb such as raptors (when gliding) and man-made sailplane gliders. This is termed soaring. However most other birds and all powered aircraft need a source of propulsion to climb. This is termed powered flight.
Animal flight | Flight | Wikipedia | 458 | 159472 | https://en.wikipedia.org/wiki/Flight | Physical sciences | Fluid mechanics | null |
The only groups of living things that use powered flight are birds, insects, and bats, while many groups have evolved gliding. The extinct pterosaurs, an order of reptiles contemporaneous with the dinosaurs, were also very successful flying animals, and there were apparently some flying dinosaurs (see Flying and gliding animals#Non-avian dinosaurs). Each of these groups' wings evolved independently, with insects the first animal group to evolve flight. The wings of the flying vertebrate groups are all based on the forelimbs, but differ significantly in structure; insect wings are hypothesized to be highly modified versions of structures that form gills in most other groups of arthropods.
Bats are the only mammals capable of sustaining level flight (see bat flight). However, there are several gliding mammals which are able to glide from tree to tree using fleshy membranes between their limbs; some can travel hundreds of meters in this way with very little loss in height. Flying frogs use greatly enlarged webbed feet for a similar purpose, and there are flying lizards which fold out their mobile ribs into a pair of flat gliding surfaces. "Flying" snakes also use mobile ribs to flatten their body into an aerodynamic shape, with a back and forth motion much the same as they use on the ground.
Flying fish can glide using enlarged wing-like fins, and have been observed soaring for hundreds of meters. It is thought that this ability was chosen by natural selection because it was an effective means of escape from underwater predators. The longest recorded flight of a flying fish was 45 seconds.
Most birds fly (see bird flight), with some exceptions. The largest birds, the ostrich and the emu, are earthbound flightless birds, as were the now-extinct dodos and the Phorusrhacids, which were the dominant predators of South America in the Cenozoic era. The non-flying penguins have wings adapted for use under water and use the same wing movements for swimming that most other birds use for flight. Most small flightless birds are native to small islands, and lead a lifestyle where flight would offer little advantage.
Among living animals that fly, the wandering albatross has the greatest wingspan, up to ; the great bustard has the greatest weight, topping at . | Flight | Wikipedia | 467 | 159472 | https://en.wikipedia.org/wiki/Flight | Physical sciences | Fluid mechanics | null |
Most species of insects can fly as adults. Insect flight makes use of either of two basic aerodynamic models: creating a leading edge vortex, found in most insects, and using clap and fling, found in very small insects such as thrips.
Many species of spiders, spider mites and lepidoptera use a technique called ballooning to ride air currents such as thermals, by exposing their gossamer threads which gets lifted by wind and atmospheric electric fields.
Mechanical
Mechanical flight is the use of a machine to fly. These machines include aircraft such as airplanes, gliders, helicopters, autogyros, airships, balloons, ornithopters as well as spacecraft. Gliders are capable of unpowered flight. Another form of mechanical flight is para-sailing, where a parachute-like object is pulled by a boat. In an airplane, lift is created by the wings; the shape of the wings of the airplane are designed specially for the type of flight desired. There are different types of wings: tempered, semi-tempered, sweptback, rectangular and elliptical. An aircraft wing is sometimes called an airfoil, which is a device that creates lift when air flows across it.
Supersonic
Supersonic flight is flight faster than the speed of sound. Supersonic flight is associated with the formation of shock waves that form a sonic boom that can be heard from the ground, and is frequently startling. The creation of this shockwave requires a significant amount of energy; because of this, supersonic flight is generally less efficient than subsonic flight at about 85% of the speed of sound.
Hypersonic
Hypersonic flight is very high speed flight where the heat generated by the compression of the air due to the motion through the air causes chemical changes to the air. Hypersonic flight is achieved primarily by reentering spacecraft such as the Space Shuttle and Soyuz.
Ballistic
Atmospheric
Some things generate little or no lift and move only or mostly under the action of momentum, gravity, air drag and in some cases thrust. This is termed ballistic flight. Examples include balls, arrows, bullets, fireworks etc.
Spaceflight
Essentially an extreme form of ballistic flight, spaceflight is the use of space technology to achieve the flight of spacecraft into and through outer space. Examples include ballistic missiles, orbital spaceflight, etc.
Spaceflight is used in space exploration, and also in commercial activities like space tourism and satellite telecommunications. Additional non-commercial uses of spaceflight include space observatories, reconnaissance satellites and other Earth observation satellites. | Flight | Wikipedia | 505 | 159472 | https://en.wikipedia.org/wiki/Flight | Physical sciences | Fluid mechanics | null |
A spaceflight typically begins with a rocket launch, which provides the initial thrust to overcome the force of gravity and propels the spacecraft from the surface of the Earth. Once in space, the motion of a spacecraft—both when unpropelled and when under propulsion—is covered by the area of study called astrodynamics. Some spacecraft remain in space indefinitely, some disintegrate during atmospheric reentry, and others reach a planetary or lunar surface for landing or impact.
Solid-state propulsion
In 2018, researchers at Massachusetts Institute of Technology (MIT) managed to fly an aeroplane with no moving parts, powered by an "ionic wind" also known as electroaerodynamic thrust.
History
Many human cultures have built devices that fly, from the earliest projectiles such as stones and spears, the
boomerang in Australia, the hot air Kongming lantern, and kites.
Aviation
George Cayley studied flight scientifically in the first half of the 19th century, and in the second half of the 19th century Otto Lilienthal made over 200 gliding flights and was also one of the first to understand flight scientifically. His work was replicated and extended by the Wright brothers who made gliding flights and finally the first controlled and extended, manned powered flights.
Spaceflight
Spaceflight, particularly human spaceflight became a reality in the 20th century following theoretical and practical breakthroughs by Konstantin Tsiolkovsky and Robert H. Goddard. The first orbital spaceflight was in 1957, and Yuri Gagarin was carried aboard the first crewed orbital spaceflight in 1961.
Physics
There are different approaches to flight. If an object has a lower density than air, then it is buoyant and is able to float in the air without expending energy. A heavier than air craft, known as an aerodyne, includes flighted animals and insects, fixed-wing aircraft and rotorcraft. Because the craft is heavier than air, it must generate lift to overcome its weight. The wind resistance caused by the craft moving through the air is called drag and is overcome by propulsive thrust except in the case of gliding.
Some vehicles also use thrust in the place of lift; for example rockets and Harrier jump jets.
Forces
Forces relevant to flight are
Propulsive thrust (except in gliders)
Lift, created by the reaction to an airflow
Drag, created by aerodynamic friction
Weight, created by gravity
Buoyancy, for lighter than air flight
These forces must be balanced for stable flight to occur.
Thrust | Flight | Wikipedia | 504 | 159472 | https://en.wikipedia.org/wiki/Flight | Physical sciences | Fluid mechanics | null |
A fixed-wing aircraft generates forward thrust when air is pushed in the direction opposite to flight. This can be done in several ways including by the spinning blades of a propeller, or a rotating fan pushing air out from the back of a jet engine, or by ejecting hot gases from a rocket engine. The forward thrust is proportional to the mass of the airstream multiplied by the difference in velocity of the airstream. Reverse thrust can be generated to aid braking after landing by reversing the pitch of variable-pitch propeller blades, or using a thrust reverser on a jet engine. Rotary wing aircraft and thrust vectoring V/STOL aircraft use engine thrust to support the weight of the aircraft, and vector sum of this thrust fore and aft to control forward speed.
Lift
In the context of an air flow relative to a flying body, the lift force is the component of the aerodynamic force that is perpendicular to the flow direction. Aerodynamic lift results when the wing causes the surrounding air to be deflected - the air then causes a force on the wing in the opposite direction, in accordance with Newton's third law of motion.
Lift is commonly associated with the wing of an aircraft, although lift is also generated by rotors on rotorcraft (which are effectively rotating wings, performing the same function without requiring that the aircraft move forward through the air). While common meanings of the word "lift" suggest that lift opposes gravity, aerodynamic lift can be in any direction. When an aircraft is cruising for example, lift does oppose gravity, but lift occurs at an angle when climbing, descending or banking. On high-speed cars, the lift force is directed downwards (called "down-force") to keep the car stable on the road.
Drag
For a solid object moving through a fluid, the drag is the component of the net aerodynamic or hydrodynamic force acting opposite to the direction of the movement. Therefore, drag opposes the motion of the object, and in a powered vehicle it must be overcome by thrust. The process which creates lift also causes some drag.
Lift-to-drag ratio | Flight | Wikipedia | 424 | 159472 | https://en.wikipedia.org/wiki/Flight | Physical sciences | Fluid mechanics | null |
Aerodynamic lift is created by the motion of an aerodynamic object (wing) through the air, which due to its shape and angle deflects the air. For sustained straight and level flight, lift must be equal and opposite to weight. In general, long narrow wings are able deflect a large amount of air at a slow speed, whereas smaller wings need a higher forward speed to deflect an equivalent amount of air and thus generate an equivalent amount of lift. Large cargo aircraft tend to use longer wings with higher angles of attack, whereas supersonic aircraft tend to have short wings and rely heavily on high forward speed to generate lift.
However, this lift (deflection) process inevitably causes a retarding force called drag. Because lift and drag are both aerodynamic forces, the ratio of lift to drag is an indication of the aerodynamic efficiency of the airplane. The lift to drag ratio is the L/D ratio, pronounced "L over D ratio." An airplane has a high L/D ratio if it produces a large amount of lift or a small amount of drag. The lift/drag ratio is determined by dividing the lift coefficient by the drag coefficient, CL/CD.
The lift coefficient Cl is equal to the lift L divided by the (density r times half the velocity V squared times the wing area A). [Cl = L / (A * .5 * r * V^2)] The lift coefficient is also affected by the compressibility of the air, which is much greater at higher speeds, so velocity V is not a linear function. Compressibility is also affected by the shape of the aircraft surfaces.
The drag coefficient Cd is equal to the drag D divided by the (density r times half the velocity V squared times the reference area A). [Cd = D / (A * .5 * r * V^2)]
Lift-to-drag ratios for practical aircraft vary from about 4:1 for vehicles and birds with relatively short wings, up to 60:1 or more for vehicles with very long wings, such as gliders. A greater angle of attack relative to the forward movement also increases the extent of deflection, and thus generates extra lift. However a greater angle of attack also generates extra drag. | Flight | Wikipedia | 458 | 159472 | https://en.wikipedia.org/wiki/Flight | Physical sciences | Fluid mechanics | null |
Lift/drag ratio also determines the glide ratio and gliding range. Since the glide ratio is based only on the relationship of the aerodynamics forces acting on the aircraft, aircraft weight will not affect it. The only effect weight has is to vary the time that the aircraft will glide for – a heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time.
Buoyancy
Air pressure acting up against an object in air is greater than the pressure above pushing down. The buoyancy, in both cases, is equal to the weight of fluid displaced - Archimedes' principle holds for air just as it does for water.
A cubic meter of air at ordinary atmospheric pressure and room temperature has a mass of about 1.2 kilograms, so its weight is about 12 newtons. Therefore, any 1-cubic-meter object in air is buoyed up with a force of 12 newtons. If the mass of the 1-cubic-meter object is greater than 1.2 kilograms (so that its weight is greater than 12 newtons), it falls to the ground when released. If an object of this size has a mass less than 1.2 kilograms, it rises in the air. Any object that has a mass that is less than the mass of an equal volume of air will rise in air - in other words, any object less dense than air will rise.
Thrust to weight ratio
Thrust-to-weight ratio is, as its name suggests, the ratio of instantaneous thrust to weight (where weight means weight at the Earth's standard acceleration ). It is a dimensionless parameter characteristic of rockets and other jet engines and of vehicles propelled by such engines (typically space launch vehicles and jet aircraft).
If the thrust-to-weight ratio is greater than the local gravity strength (expressed in gs), then flight can occur without any forward motion or any aerodynamic lift being required.
If the thrust-to-weight ratio times the lift-to-drag ratio is greater than local gravity then takeoff using aerodynamic lift is possible.
Flight dynamics
Flight dynamics is the science of air and space vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of mass, known as pitch, roll and yaw (See Tait-Bryan rotations for an explanation). | Flight | Wikipedia | 481 | 159472 | https://en.wikipedia.org/wiki/Flight | Physical sciences | Fluid mechanics | null |
The control of these dimensions can involve a horizontal stabilizer (i.e. "a tail"), ailerons and other movable aerodynamic devices which control angular stability i.e. flight attitude (which in turn affects altitude, heading). Wings are often angled slightly upwards- they have "positive dihedral angle" which gives inherent roll stabilization.
Energy efficiency
To create thrust so as to be able to gain height, and to push through the air to overcome the drag associated with lift all takes energy. Different objects and creatures capable of flight vary in the efficiency of their muscles, motors and how well this translates into forward thrust.
Propulsive efficiency determines how much energy vehicles generate from a unit of fuel.
Range
The range that powered flight articles can achieve is ultimately limited by their drag, as well as how much energy they can store on board and how efficiently they can turn that energy into propulsion.
For powered aircraft the useful energy is determined by their fuel fraction- what percentage of the takeoff weight is fuel, as well as the specific energy of the fuel used.
Power-to-weight ratio
All animals and devices capable of sustained flight need relatively high power-to-weight ratios to be able to generate enough lift and/or thrust to achieve take off.
Takeoff and landing
Vehicles that can fly can have different ways to takeoff and land. Conventional aircraft accelerate along the ground until sufficient lift is generated for takeoff, and reverse the process for landing. Some aircraft can take off at low speed; this is called a short takeoff. Some aircraft such as helicopters and Harrier jump jets can take off and land vertically. Rockets also usually take off and land vertically, but some designs can land horizontally.
Guidance, navigation and control
Navigation
Navigation is the systems necessary to calculate current position (e.g. compass, GPS, LORAN, star tracker, inertial measurement unit, and altimeter).
In aircraft, successful air navigation involves piloting an aircraft from place to place without getting lost, breaking the laws applying to aircraft, or endangering the safety of those on board or on the ground. | Flight | Wikipedia | 421 | 159472 | https://en.wikipedia.org/wiki/Flight | Physical sciences | Fluid mechanics | null |
The techniques used for navigation in the air will depend on whether the aircraft is flying under the visual flight rules (VFR) or the instrument flight rules (IFR). In the latter case, the pilot will navigate exclusively using instruments and radio navigation aids such as beacons, or as directed under radar control by air traffic control. In the VFR case, a pilot will largely navigate using dead reckoning combined with visual observations (known as pilotage), with reference to appropriate maps. This may be supplemented using radio navigation aids.
Guidance
A guidance system is a device or group of devices used in the navigation of a ship, aircraft, missile, rocket, satellite, or other moving object. Typically, guidance is responsible for the calculation of the vector (i.e., direction, velocity) toward an objective.
Control
A conventional fixed-wing aircraft flight control system consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft engine controls are also considered as flight controls as they change speed.
Traffic
In the case of aircraft, air traffic is controlled by air traffic control systems.
Collision avoidance is the process of controlling spacecraft to try to prevent collisions.
Flight safety
Air safety is a term encompassing the theory, investigation and categorization of flight failures, and the prevention of such failures through regulation, education and training. It can also be applied in the context of campaigns that inform the public as to the safety of air travel. | Flight | Wikipedia | 305 | 159472 | https://en.wikipedia.org/wiki/Flight | Physical sciences | Fluid mechanics | null |
RGB color spaces is a category of additive colorimetric color spaces specifying part of its absolute color space definition using the RGB color model.
RGB color spaces are commonly found describing the mapping of the RGB color model to human perceivable color, but some RGB color spaces use imaginary (non-real-world) primaries and thus can not be displayed directly.
Like any color space, while the specifications in this category use the RGB color model to describe their space, it is not mandatory to use that model to signal pixel color values. Broadcast TV color spaces like NTSC, PAL, Rec. 709, Rec. 2020 additionally describe a translation from RGB to YCbCr and that is how they are usually signalled for transmission, but an image can be stored as either RGB or YCbCr. This demonstrates using the singular term "RGB color space" can be misleading, since a chosen color space or signalled colour can be described by any appropriate color model. However the singular can be seen in specifications where storage signalled as RGB is its intended use.
Definition
The normal human eye contains three types of color-sensitive cone cells. Each cell is responsive to light of either long, medium, or short wavelengths, which we generally categorize as red, green, and blue. Taken together, the responses of these cone cells are called the Tristimulus values, and the combination of their responses is processed into the psychological effect of color vision.
RGB use in color space definitions employ primaries (and often a white point) based on the RGB color model, to map to real world color. Applying Grassmann's law of light additivity, the range of colors that can be produced are those enclosed within the triangle on the chromaticity diagram defined using the primaries as vertices.
The primary colors are usually mapped to xyY chromaticity coordinates, though the uʹ,vʹ coordinates from the UCS chromaticity diagram may be used. Both xyY and uʹ,vʹ are derived from the CIE 1931 color space, a device independent space also known as XYZ which covers the full gamut of human-perceptible colors visible to the CIE 2° standard observer.
Applications | RGB color spaces | Wikipedia | 459 | 159506 | https://en.wikipedia.org/wiki/RGB%20color%20spaces | Physical sciences | Basics | Physics |
RGB color spaces are well-suited to describing the electronic display of color, such as computer monitors and color television. These devices often reproduce colours using an array of red, green, and blue phosphors agitated by a cathode-ray tube (CRT), or an array of red, green, and blue LCDs lit by a backlight, and are therefore naturally described by an additive color model with RGB primaries.
Early examples of RGB color spaces came with the adoption of the NTSC color television standard in 1953 across North America, followed by PAL and SECAM covering the rest of the world. These early RGB spaces were defined in part by the phosphor used by CRTs in use at the time, and the gamma of the electron beam. While these color spaces reproduced the intended colors using additive red, green, and blue primaries, the broadcast signal itself was encoded from RGB components to a composite signal such as YIQ, and decoded back by the receiver into RGB signals for display.
HDTV uses the BT.709 color space, later repurposed for computer monitors as sRGB. Both use the same color primaries and white point, but different transfer functions, as HDTV is intended for a dark living room while sRGB is intended for a brighter office environment. The gamut of these spaces is limited, covering only 35.9% of the CIE 1931 gamut. While this allows the use of a limited bit depth without causing color banding, and therefore reduces transmission bandwidth, it also prevents the encoding of deeply saturated colors that might be available in an alternate color spaces. Some RGB color spaces such as Adobe RGB and ProPhoto intended for the creation, rather than transmission, of images are designed with expanded gamuts to address this issue, however this does not mean the larger space has 'more colors". The numerical quantity of colors is related to bit depth and not the size or shape of the gamut. A large space with a low bit depth can be detrimental to the gamut density and result in high errors.
More recent color spaces such as Rec. 2020 for UHD-TVs define an extremely large gamut covering 63.3% of the CIE 1931 space. This standard is not currently realisable with current LCD technology, and alternative architectures such as quantum dot or OLED based devices are currently in development.
Color space specifications employing the RGB color model | RGB color spaces | Wikipedia | 500 | 159506 | https://en.wikipedia.org/wiki/RGB%20color%20spaces | Physical sciences | Basics | Physics |
The CIE 1931 color space standard defines both the CIE RGB space, which is a color space with monochromatic primaries, and the CIE XYZ color space, which is functionally similar to a linear RGB color space, however the primaries are not physically realizable, thus are not described as red, green, and blue.
M.A.C. is not to be confused with MacOS. Here, M.A.C.refers to Multiplexed Analogue Components. | RGB color spaces | Wikipedia | 102 | 159506 | https://en.wikipedia.org/wiki/RGB%20color%20spaces | Physical sciences | Basics | Physics |
Barnacles are arthropods of the subclass Cirripedia in the subphylum Crustacea. They are related to crabs and lobsters, with similar nauplius larvae. Barnacles are exclusively marine invertebrates; many species live in shallow and tidal waters. Some 2,100 species have been described.
Barnacle adults are sessile; most are suspension feeders with hard calcareous shells, but the Rhizocephala are specialized parasites of other crustaceans, with reduced bodies. Barnacles have existed since at least the mid-Carboniferous, some 325 million years ago.
In folklore, barnacle geese were once held to emerge fully formed from goose barnacles. Both goose barnacles and the Chilean giant barnacle are fished and eaten. Barnacles are economically significant as biofouling on ships, where they cause hydrodynamic drag, reducing efficiency.
Etymology
The word "barnacle" is attested in the early 13th century as Middle English "bernekke" or "bernake", close to Old French "bernaque" and medieval Latin bernacae or berneka, denoting the barnacle goose. Because the full life cycles of both barnacles and geese were unknown at the time, (geese spend their breeding seasons in the Arctic) a folktale emerged that geese hatched from barnacles. It was not applied strictly to the arthropod until the 1580s. The ultimate meaning of the word is unknown.
The name comes from the Latin words cirritus "curly" from cirrus "curl" and pedis from pes "foot". The two words together mean "curly-footed", alluding to the curved legs used in filter-feeding.
Description
Most barnacles are encrusters, attaching themselves to a hard substrate such as a rock, the shell of a mollusc, or a ship; or to an animal such as a whale (whale barnacles). The most common form, acorn barnacles, are sessile, growing their shells directly onto the substrate, whereas goose barnacles attach themselves by means of a stalk.
Anatomy and physiology | Barnacle | Wikipedia | 452 | 159512 | https://en.wikipedia.org/wiki/Barnacle | Biology and health sciences | Crustaceans | Animals |
Barnacles have a carapace made of six hard calcareous plates, with a lid or operculum made of four more plates. Inside the carapace, the animal lies on its stomach, projecting its limbs downwards. Segmentation is usually indistinct; the body is more or less evenly divided between the head and thorax, with little or no abdomen. Adult barnacles have few appendages on their heads, with only a single, vestigial pair of antennae attached to the cement gland. The six pairs of thoracic limbs are called cirri; these are feathery and very long. The cirri extend to filter food, such as plankton, from the water and move it towards the mouth.
Acorn barnacles are attached to the substratum by cement glands that form the base of the first pair of antennae; in effect, the animal is fixed upside down by means of its forehead. In some barnacles, the cement glands are fixed to a long, muscular stalk, but in most they are part of a flat membrane or calcified plate. These glands secrete a type of natural quick cement made of complex protein bonds (polyproteins) and other trace components like calcium. This natural cement can withstand a pulling strength of and a sticking strength of .
Barnacles have no true heart, although a sinus close to the esophagus performs a similar function, with blood being pumped through it by a series of muscles. The blood vascular system is minimal. Similarly, they have no gills, absorbing oxygen from the water through the cirri and the surface of the body. The excretory organs of barnacles are maxillary glands.
The main sense of barnacles appears to be touch, with the hairs on the limbs being especially sensitive. The adult has three photoreceptors (ocelli), one median and two lateral. These record the stimulus for the barnacle shadow reflex, where a sudden decrease in light causes cessation of the fishing rhythm and closing of the opercular plates. The photoreceptors are likely only capable of sensing the difference between light and dark. This eye is derived from the primary naupliar eye.
Life cycle
Barnacles pass through two distinct larval stages, the nauplius and the cyprid, before developing into a mature adult.
Nauplius larva | Barnacle | Wikipedia | 493 | 159512 | https://en.wikipedia.org/wiki/Barnacle | Biology and health sciences | Crustaceans | Animals |
A fertilised egg hatches into a nauplius: a one-eyed larva comprising a head and a telson with three pairs of limbs, lacking a thorax or abdomen. This undergoes six moults, passing through five instars, before transforming into the cyprid stage. Nauplii are typically initially brooded by the parent, and released after the first moult as larvae that swim freely using setae. All but the first instars are filter feeders.
Cypris larva
The cypris larva is the second and final larval stage before adulthood. In Rhizocephala and Thoracica an abdomen is absent in this stage, but the y-cyprids (post-naupliar instar) has three distinct abdominal segments. It is not a feeding stage; its role is to find a suitable place to settle, since the adults are sessile. The cyprid stage lasts from days to weeks. It explores potential surfaces with modified antennules; once it has found a suitable spot, it attaches head-first using its antennules and a secreted glycoproteinous cement. Larvae assess surfaces based upon their surface texture, chemistry, relative wettability, color, and the presence or absence and composition of a surface biofilm; swarming species are more likely to attach near other barnacles. As the larva exhausts its energy reserves, it becomes less selective in the sites it selects. It cements itself permanently to the substrate with another proteinaceous compound, and then undergoes metamorphosis into a juvenile barnacle.
Adult
Typical acorn barnacles develop six hard calcareous plates to surround and protect their bodies. For the rest of their lives, they are cemented to the substrate, using their feathery legs (cirri) to capture plankton. Once metamorphosis is over and they have reached their adult form, barnacles continue to grow by adding new material to their heavily calcified plates. These plates are not moulted; however, like all ecdysozoans, the barnacle moults its cuticle.
Sexual reproduction | Barnacle | Wikipedia | 453 | 159512 | https://en.wikipedia.org/wiki/Barnacle | Biology and health sciences | Crustaceans | Animals |
Most barnacles are hermaphroditic, producing both eggs and sperms. A few species have separate sexes, or have both males and hermaphrodites. The ovaries are located in the base or stalk, and may extend into the mantle, while the testes are towards the back of the head, often extending into the thorax. Typically, recently moulted hermaphroditic individuals are receptive as females. Self-fertilization, although theoretically possible, has been experimentally shown to be rare in barnacles.
The sessile lifestyle of acorn barnacles makes sexual reproduction difficult, as they cannot leave their shells to mate. To facilitate genetic transfer between isolated individuals, barnacles have developed extraordinarily long penises. Barnacles are believed to have the largest penis-to-body size ratio of any known animal, up to eight times their body length, though on exposed coasts the penis is shorter and thicker. The mating of acorn barnacles is described as pseudocopulation.
The goose barnacle Pollicipes polymerus can alternatively reproduce by spermcasting, in which the male barnacle releases his sperm into the water, to be taken up by females. Isolated individuals always made use of spermcasting and sperm capture, as did a quarter of individuals with a close neighbour. This 2013 discovery overturned the long-held belief that barnacles were limited to pseucocopulation or hermaphroditism.
Rhizocephalan barnacles had been considered hermaphroditic, but their males inject themselves into females' bodies, degrading to little more than sperm-producing cells.
Ecology
Filter feeding
Most barnacles are filter feeders. From within their shell, they repeatedly reach into the water column with their cirri. These feathery appendages beat rhythmically to draw plankton and detritus into the shell for consumption.
Species-specific zones
Although they have been found at water depths to , most barnacles inhabit shallow waters, with 75% of species living in water depths less than , and 25% inhabiting the intertidal zone. Within the intertidal zone, different species of barnacles live in very tightly constrained locations, allowing the exact height of an assemblage above or below sea level to be precisely determined. | Barnacle | Wikipedia | 486 | 159512 | https://en.wikipedia.org/wiki/Barnacle | Biology and health sciences | Crustaceans | Animals |
Since the intertidal zone periodically desiccates, barnacles are well adapted against water loss. Their calcite shells are impermeable, and they can close their apertures with movable plates when not feeding. Their hard shells are assumed by zoologists to have evolved as an anti-predator adaptation.
One group of stalked barnacles has adapted to a rafting lifestyle, drifting around close to the water's surface. They colonize every floating object, such as driftwood, and like some non-stalked barnacles attach themselves to marine animals. The species most specialized for this lifestyle is Dosima fascicularis, which secretes a gas-filled cement that makes it float at the surface.
Parasitism
Other members of the class have an entirely different mode of life. Barnacles of the superorder Rhizocephala, including the genus Sacculina, are parasitic castrators of other arthropods, including crabs. The anatomy of these parasitic barnacles is greatly reduced compared to their free-living relatives. They have no carapace or limbs, having only unsegmented sac-like bodies. They feed by extending thread-like rhizomes of living cells into their hosts' bodies from their points of attachment.
Goose barnacles of the genus Anelasma (in the order Pollicipedomorpha) are specialized parasites of certain shark species. Their cirri are no longer used to filter-feed. Instead, these barnacles get their nutrients directly from the host through a root-like body part embedded in the shark's flesh.
Competitors
Barnacles are displaced by limpets and mussels, which compete for space. They employ two strategies to overwhelm their competitors: "swamping", and fast growth. In the swamping strategy, vast numbers of barnacles settle in the same place at once, covering a large patch of substrate, allowing at least some to survive in the balance of probabilities. Fast growth allows the suspension feeders to access higher levels of the water column than their competitors, and to be large enough to resist displacement; species employing this response, such as the aptly named Megabalanus, can reach in length. | Barnacle | Wikipedia | 461 | 159512 | https://en.wikipedia.org/wiki/Barnacle | Biology and health sciences | Crustaceans | Animals |
Competitors may include other barnacles. Balanoids gained their advantage over the chthalamoids in the Oligocene, when they evolved tubular skeletons, which provide better anchorage to the substrate, and allow them to grow faster, undercutting, crushing, and smothering chthalamoids.
Predators and parasites
Among the most common predators of barnacles are whelks. They are able to grind through the calcareous exoskeleton and eat the animal inside. Barnacle larvae are consumed by filter-feeding benthic predators including the mussel Mytilus edulis and the ascidian Styela gibbsi. Another predator is the starfish species Pisaster ochraceus. A stalked barnacle in the Iblomorpha, Chaetolepas calcitergum, lacks a heavily mineralised shell, but contains a high concentration of toxic bromine; this may serve to deter predators. The turbellarian flatworm Stylochus, a serious predator of oyster spat, has been found in barnacles. Parasites of barnacles include many species of Gregarinasina (alveolate protozoa), a few fungi, a few species of trematodes, and a parasitic castrator isopod, Hemioniscus balani.
History of taxonomy
Barnacles were classified by Linnaeus and Cuvier as Mollusca, but in 1830 John Vaughan Thompson published observations showing the metamorphosis of the nauplius and cypris larvae into adult barnacles, and noted that these larvae were similar to those of crustaceans. In 1834, Hermann Burmeister reinterpreted these findings, moving barnacles from the Mollusca to Articulata (in modern terms, annelids + arthropods), showing naturalists that detailed study was needed to reevaluate their taxonomy. | Barnacle | Wikipedia | 387 | 159512 | https://en.wikipedia.org/wiki/Barnacle | Biology and health sciences | Crustaceans | Animals |
Charles Darwin took up this challenge in 1846, and developed his initial interest into a major study published as a series of monographs in 1851 and 1854. He undertook this study at the suggestion of his friend the botanist Joseph Dalton Hooker, namely to thoroughly understand at least one species before making the generalisations needed for his theory of evolution by natural selection.
The Royal Society notes that barnacles occupied Darwin, who worked from home, so intensely "that his son assumed all fathers behaved the same way: when visiting a friend he asked, 'Where does your father do his barnacles?'" Upon the conclusion of his research, Darwin declared "I hate a barnacle as no man ever did before."
Evolution
Fossil record
The oldest definitive fossil barnacle is Praelepas from the mid-Carboniferous, around 330-320 million years ago. Older claimed barnacles such as Priscansermarinus from the Middle Cambrian, some , do not show clear barnacle morphological traits, though Rhamphoverritor from the Silurian Coalbrookdale Formation of England may represent a stem-group barnacle. Barnacles first radiated and became diverse during the Late Cretaceous. Barnacles underwent a second, much larger radiation beginning during the Neogene and still continuing.
Phylogeny
The following cladogram, not fully resolved, shows the phylogenetic relationships of the Cirripedia within Thecostraca as of 2021.
Taxonomy
Over 2,100 species of Cirripedia have been described. Some authorities regard the Cirripedia as a full class or subclass. In 2001, Martin and Davis placed Cirripedia as an infraclass of Thecostraca, and divided it into six orders:
Infraclass Cirripedia Burmeister, 1834
Superorder Acrothoracica Gruvel, 1905
Order Pygophora Berndt, 1907
Order Apygophora Berndt, 1907
Superorder Rhizocephala Müller, 1862
Order Kentrogonida Delage, 1884
Order Akentrogonida Häfele, 1911
Superorder Thoracica Darwin, 1854
Order Pedunculata Lamarck, 1818
Order Sessilia Lamarck, 1818 | Barnacle | Wikipedia | 457 | 159512 | https://en.wikipedia.org/wiki/Barnacle | Biology and health sciences | Crustaceans | Animals |
In 2021, Chan et al. elevated Cirripedia to a subclass of the Thecostraca, and the superorders Acrothoracica, Rhizocephala, and Thoracica to infraclass. The updated classification with 11 orders has been accepted in the World Register of Marine Species.
Subclass Cirripedia Burmeister, 1834
Infraclass Acrothoracica Gruvel, 1905
Order Cryptophialida Kolbasov, Newman & Hoeg, 2009
Order Lithoglyptida Kolbasov, Newman & Hoeg, 2009
Infraclass Rhizocephala Müller, 1862
Infraclass Thoracica Darwin, 1854
Superorder Phosphatothoracica Gale, 2019
Order Iblomorpha Buckeridge & Newman, 2006
Order † Eolepadomorpha Chan et al., 2021
Superorder Thoracicalcarea Gale, 2015
Order Calanticomorpha Chan et al., 2021
Order Pollicipedomorpha Chan et al., 2021
Order Scalpellomorpha Buckeridge & Newman, 2006
Order † Archaeolepadomorpha Chan et al., 2021
Order † Brachylepadomorpha Withers, 1923
(Unranked) Sessilia
Order Balanomorpha Pilsbry, 1916
Order Verrucomorpha Pilsbry, 1916
Relationship with humans
Biofouling
Barnacles are of economic consequence, as they often attach themselves to man-made structures. Particularly in the case of ships, they are classified as fouling organisms. The number and size of barnacles that cover ships can impair their efficiency by causing hydrodynamic drag.
As food
The flesh of some barnacles is routinely consumed by humans, including Japanese goose barnacles (e.g. Capitulum mitella), and goose barnacles (e.g. Pollicipes pollicipes), a delicacy in Spain and Portugal. The Chilean giant barnacle Austromegabalanus psittacus is fished, or overfished, in commercial quantities on the Chilean coast, where it is known as the .
Technological applications
MIT researchers have developed an adhesive inspired by the protein-based bioglue produced by barnacles to firmly attach to rocks. The adhesive can form a tight seal to halt bleeding within about 15 seconds of application. | Barnacle | Wikipedia | 503 | 159512 | https://en.wikipedia.org/wiki/Barnacle | Biology and health sciences | Crustaceans | Animals |
The stable isotope signals in the layers of barnacle shells can potentially be used as a forensic tracking method for whales, loggerhead turtles and for marine debris, such as shipwrecks or aircraft wreckage.
In culture
One version of the barnacle goose myth is that the birds emerge fully formed from goose barnacles. The myth, with variants such as that the goose barnacles grow on trees, owes its longstanding popularity to ignorance of bird migration. The myth survived to modern times through bestiaries.
More recently, Barnacle Bill became a "comic folktype" of a seaman, with a drinking song and several films (a 1930 animated short with Betty Boop, a 1935 British drama, a 1941 feature with Wallace Beery, and a 1957 Ealing comedy) named after him.
The political reformer John W. Gardner likened middle managers who settle into a comfortable position and "have stopped learning or growing" to the barnacle, who "is confronted with an existential decision about where it's going to live. Once it decides... it spends the rest of its life with its head cemented to a rock". | Barnacle | Wikipedia | 232 | 159512 | https://en.wikipedia.org/wiki/Barnacle | Biology and health sciences | Crustaceans | Animals |
The muscular system is an organ system consisting of skeletal, smooth, and cardiac muscle. It permits movement of the body, maintains posture, and circulates blood throughout the body. The muscular systems in vertebrates are controlled through the nervous system although some muscles (such as the cardiac muscle) can be completely autonomous. Together with the skeletal system in the human, it forms the musculoskeletal system, which is responsible for the movement of the body.
Types
There are three distinct types of muscle: skeletal muscle, cardiac or heart muscle, and smooth (non-striated) muscle. Muscles provide strength, balance, posture, movement, and heat for the body to keep warm.
There are more than 600 muscles in an adult male human body. A kind of elastic tissue makes up each muscle, which consists of thousands, or tens of thousands, of small muscle fibers. Each fiber comprises many tiny strands called fibrils, impulses from nerve cells control the contraction of each muscle fiber.
Skeletal
Skeletal muscle, is a type of striated muscle, composed of muscle cells, called muscle fibers, which are in turn composed of myofibrils. Myofibrils are composed of sarcomeres, the basic building blocks of striated muscle tissue. Upon stimulation by an action potential, skeletal muscles perform a coordinated contraction by shortening each sarcomere. The best proposed model for understanding contraction is the sliding filament model of muscle contraction. Within the sarcomere, actin and myosin fibers overlap in a contractile motion towards each other. Myosin filaments have club-shaped myosin heads that project toward the actin filaments, and provide attachment points on binding sites for the actin filaments. The myosin heads move in a coordinated style; they swivel toward the center of the sarcomere, detach and then reattach to the nearest active site of the actin filament. This is called a ratchet type drive system. | Muscular system | Wikipedia | 412 | 159748 | https://en.wikipedia.org/wiki/Muscular%20system | Biology and health sciences | Muscular system | null |
This process consumes large amounts of adenosine triphosphate (ATP), the energy source of the cell. ATP binds to the cross-bridges between myosin heads and actin filaments. The release of energy powers the swiveling of the myosin head. When ATP is used, it becomes adenosine diphosphate (ADP), and since muscles store little ATP, they must continuously replace the discharged ADP with ATP. Muscle tissue also contains a stored supply of a fast-acting recharge chemical, creatine phosphate, which when necessary can assist with the rapid regeneration of ADP into ATP.
Calcium ions are required for each cycle of the sarcomere. Calcium is released from the sarcoplasmic reticulum into the sarcomere when a muscle is stimulated to contract. This calcium uncovers the actin-binding sites. When the muscle no longer needs to contract, the calcium ions are pumped from the sarcomere and back into storage in the sarcoplasmic reticulum.
There are approximately 639 skeletal muscles in the human body.
Cardiac
Heart muscle is striated muscle but is distinct from skeletal muscle because the muscle fibers are laterally connected. Furthermore, just as with smooth muscles, their movement is involuntary. Heart muscle is controlled by the sinus node influenced by the autonomic nervous system.
Smooth
Smooth muscle contraction is regulated by the autonomic nervous system, hormones, and local chemical signals, allowing for gradual and sustained contractions. This type of muscle tissue is also capable of adapting to different levels of stretch and tension, which is important for maintaining proper blood flow and the movement of materials through the digestive system.
Physiology | Muscular system | Wikipedia | 348 | 159748 | https://en.wikipedia.org/wiki/Muscular%20system | Biology and health sciences | Muscular system | null |
Contraction
Neuromuscular junctions are the focal point where a motor neuron attaches to a muscle. Acetylcholine, (a neurotransmitter used in skeletal muscle contraction) is released from the axon terminal of the nerve cell when an action potential reaches the microscopic junction called a synapse. A group of chemical messengers across the synapse and stimulate the formation of electrical changes, which are produced in the muscle cell when the acetylcholine binds to receptors on its surface. Calcium is released from its storage area in the cell's sarcoplasmic reticulum. An impulse from a nerve cell causes calcium release and brings about a single, short muscle contraction called a muscle twitch. If there is a problem at the neuromuscular junction, a very prolonged contraction may occur, such as the muscle contractions that result from tetanus. Also, a loss of function at the junction can produce paralysis.
Skeletal muscles are organized into hundreds of motor units, each of which involves a motor neuron, attached by a series of thin finger-like structures called axon terminals. These attach to and control discrete bundles of muscle fibers. A coordinated and fine-tuned response to a specific circumstance will involve controlling the precise number of motor units used. While individual muscle units' contract as a unit, the entire muscle can contract on a predetermined basis due to the structure of the motor unit. Motor unit coordination, balance, and control frequently come under the direction of the cerebellum of the brain. This allows for complex muscular coordination with little conscious effort, such as when one drives a car without thinking about the process.
Tendon
A tendon is a piece of connective tissue that connects a muscle to a bone. When a muscle intercepts, it pulls against the skeleton to create movement. A tendon connects this muscle to a bone, making this function possible.
Aerobic and anaerobic muscle activity
At rest, the body produces the majority of its ATP aerobically in the mitochondria without producing lactic acid or other fatiguing byproducts. During exercise, the method of ATP production varies depending on the fitness of the individual as well as the duration and intensity of exercise. At lower activity levels, when exercise continues for a long duration (several minutes or longer), energy is produced aerobically by combining oxygen with carbohydrates and fats stored in the body. | Muscular system | Wikipedia | 501 | 159748 | https://en.wikipedia.org/wiki/Muscular%20system | Biology and health sciences | Muscular system | null |
During activity that is higher in intensity, with possible duration decreasing as intensity increases, ATP production can switch to anaerobic pathways, such as the use of the creatine phosphate and the phosphagen system or anaerobic glycolysis. Aerobic ATP production is biochemically much slower and can only be used for long-duration, low-intensity exercise, but produces no fatiguing waste products that cannot be removed immediately from the sarcomere and the body, and it results in a much greater number of ATP molecules per fat or carbohydrate molecule. Aerobic training allows the oxygen delivery system to be more efficient, allowing aerobic metabolism to begin quicker. Anaerobic ATP production produces ATP much faster and allows near-maximal intensity exercise, but also produces significant amounts of lactic acid which render high-intensity exercise unsustainable for more than several minutes. The phosphagen system is also anaerobic. It allows for the highest levels of exercise intensity, but intramuscular stores of phosphocreatine are very limited and can only provide energy for exercises lasting up to ten seconds. Recovery is very quick, with full creatine stores regenerated within five minutes.
Clinical significance
Multiple diseases can affect the muscular system.
Muscular Dystrophy
Muscular dystrophy is a group of disorders associated with progressive muscle weakness and loss of muscle mass. These disorders are caused by mutations in a person's genes. The disease affects between 19.8 and 25.1 per 100,000 person-years globally.
There are more than 30 types of muscular dystrophy. Depending on the type, muscular dystrophy can affect the patient's heart and lungs, and/or their ability to move, walk, and perform daily activities. The most common types include:
Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD)
Myotonic dystrophy
Limb-Girdle (LGMD)
Facioscapulohumeral dystrophy (FSHD)
Congenital dystrophy (CMD)
Distal (DD)
Oculopharyngeal dystrophy (OPMD)
Emery-Dreifuss (EDMD) | Muscular system | Wikipedia | 468 | 159748 | https://en.wikipedia.org/wiki/Muscular%20system | Biology and health sciences | Muscular system | null |
Pectin ( : "congealed" and "curdled") is a heteropolysaccharide, a structural polymer contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal chemical component of pectin is galacturonic acid (a sugar acid derived from galactose) which was isolated and described by Henri Braconnot in 1825. Commercially produced pectin is a white-to-light-brown powder, produced from citrus fruits for use as an edible gelling agent, especially in jams and jellies, dessert fillings, medications, and sweets; as a food stabiliser in fruit juices and milk drinks, and as a source of dietary fiber.
Biology
Pectin is composed of complex polysaccharides that are present in the primary cell walls of a plant, and are abundant in the green parts of terrestrial plants.
Pectin is the principal component of the middle lamella, where it binds cells. Pectin is deposited by exocytosis into the cell wall via vesicles produced in the Golgi apparatus. The amount, structure and chemical composition of pectin is different among plants, within a plant over time, and in various parts of a plant. Pectin is an important cell wall polysaccharide that allows primary cell wall extension and plant growth. During fruit ripening, pectin is broken down by the enzymes pectinase and pectinesterase, in which process the fruit becomes softer as the middle lamellae break down and cells become separated from each other. A similar process of cell separation caused by the breakdown of pectin occurs in the abscission zone of the petioles of deciduous plants at leaf fall.
Pectin is a natural part of the human diet, but does not contribute significantly to nutrition. The daily intake of pectin from fruits and vegetables can be estimated to be around 5 g if approximately 500 g of fruits and vegetables are consumed per day.
In human digestion, pectin binds to cholesterol in the gastrointestinal tract and slows glucose absorption by trapping carbohydrates. Pectin is thus a soluble dietary fiber. In non-obese diabetic (NOD) mice pectin has been shown to increase the incidence of autoimmune type 1 diabetes. | Pectin | Wikipedia | 498 | 159750 | https://en.wikipedia.org/wiki/Pectin | Biology and health sciences | Carbohydrates | Biology |
A study found that after consumption of fruit the concentration of methanol in the human body increased by as much as an order of magnitude due to the degradation of natural pectin (which is esterified with methanol) in the colon.
Pectin has been observed to have some function in repairing the DNA of some types of plant seeds, usually desert plants. Pectinaceous surface pellicles, which are rich in pectin, create a mucilage layer that holds in dew that helps the cell repair its DNA.
Consumption of pectin has been shown to slightly (3–7%) reduce blood LDL cholesterol levels. The effect depends upon the source of pectin; apple and citrus pectins were more effective than orange pulp fibre pectin. The mechanism appears to be an increase of viscosity in the intestinal tract, leading to a reduced absorption of cholesterol from bile or food. In the large intestine and colon, microorganisms degrade pectin and liberate short-chain fatty acids that have a positive prebiotic effect.
Chemistry
Pectins, also known as pectic polysaccharides, are rich in galacturonic acid. Several distinct polysaccharides have been identified and characterised within the pectic group. Homogalacturonans are linear chains of α-(1–4)-linked D-galacturonic acid. Substituted galacturonans are characterised by the presence of saccharide appendant residues (such as D-xylose or D-apiose in the respective cases of xylogalacturonan and apiogalacturonan) branching from a backbone of D-galacturonic acid residues. Rhamnogalacturonan I pectins (RG-I) contain a backbone of the repeating disaccharide: 4)-α-D-galacturonic acid-(1,2)-α-L-rhamnose-(1. From many of the rhamnose residues, sidechains of various neutral sugars branch off. The neutral sugars are mainly D-galactose, L-arabinose and D-xylose, with the types and proportions of neutral sugars varying with the origin of pectin. | Pectin | Wikipedia | 487 | 159750 | https://en.wikipedia.org/wiki/Pectin | Biology and health sciences | Carbohydrates | Biology |
Another structural type of pectin is rhamnogalacturonan II (RG-II), which is a less frequent, complex, highly branched polysaccharide. Rhamnogalacturonan II is classified by some authors within the group of substituted galacturonans since the rhamnogalacturonan II backbone is made exclusively of D-galacturonic acid units.
The molecular weight of isolated pectine greatly varies by the source and the method of isolation. Values have been reported as low as 28 kDa for apple pomace up to 753 kDa for sweet potato peels.
In nature, around 80 percent of carboxyl groups of galacturonic acid are esterified with methanol. This proportion is decreased to a varying degree during pectin extraction. Pectins are classified as high- versus low-methoxy pectins (short HM-pectins versus LM-pectins), with more or less than half of all the galacturonic acid esterified. The ratio of esterified to non-esterified galacturonic acid determines the behaviour of pectin in food applications – HM-pectins can form a gel under acidic conditions in the presence of high sugar concentrations, while LM-pectins form gels by interaction with divalent cations, particularly Ca2+, according to the idealized 'egg box' model, in which ionic bridges are formed between calcium ions and the ionised carboxyl groups of the galacturonic acid.
In high-methoxy pectins at soluble solids content above 60% and a pH value between 2.8 and 3.6, hydrogen bonds and hydrophobic interactions bind the individual pectin chains together. These bonds form as water is bound by sugar and forces pectin strands to stick together. These form a three-dimensional molecular net that creates the macromolecular gel. The gelling-mechanism is called a low-water-activity gel or sugar-acid-pectin gel.
While low-methoxy pectins need calcium to form a gel, they can do so at lower soluble solids and higher pH than high-methoxy pectins. Normally low-methoxy pectins form gels with a range of pH from 2.6 to 7.0 and with a soluble solids content between 10 and 70%. | Pectin | Wikipedia | 503 | 159750 | https://en.wikipedia.org/wiki/Pectin | Biology and health sciences | Carbohydrates | Biology |
The non-esterified galacturonic acid units can be either free acids (carboxyl groups) or salts with sodium, potassium, or calcium. The salts of partially esterified pectins are called pectinates, if the degree of esterification is below 5 percent the salts are called pectates, the insoluble acid form, pectic acid.
Some plants, such as sugar beet, potatoes and pears, contain pectins with acetylated galacturonic acid in addition to methyl esters. Acetylation prevents gel-formation but increases the stabilising and emulsifying effects of pectin.
Amidated pectin is a modified form of pectin. Here, some of the galacturonic acid is converted with ammonia to carboxylic acid amide. These pectins are more tolerant of varying calcium concentrations that occur in use.
Thiolated pectin exhibits substantially improved gelling properties since this thiomer is able to crosslink via disulfide bond formation. These high gelling properties are advantageous for various pharmaceutical applications and applications in food industry.
To prepare a pectin-gel, the ingredients are heated, dissolving the pectin. Upon cooling below gelling temperature, a gel starts to form. If gel formation is too strong, syneresis or a granular texture are the result, while weak gelling leads to excessively soft gels.
Amidated pectins behave like low-ester pectins but need less calcium and are more tolerant of excess calcium. Also, gels from amidated pectin are thermoreversible; they can be heated and after cooling solidify again, whereas conventional pectin-gels will afterwards remain liquid.
High-ester pectins set at higher temperatures than low-ester pectins. However, gelling reactions with calcium increase as the degree of esterification falls. Similarly, lower pH-values or higher soluble solids (normally sugars) increase gelling speeds. Suitable pectins can therefore be selected for jams and jellies, or for higher-sugar confectionery jellies.
Sources and production
Pears, apples, guavas, quince, plums, gooseberries, and oranges and other citrus fruits contain large amounts of pectin, while soft fruits, like cherries, grapes, and strawberries, contain small amounts of pectin. | Pectin | Wikipedia | 512 | 159750 | https://en.wikipedia.org/wiki/Pectin | Biology and health sciences | Carbohydrates | Biology |
Typical levels of pectin in fresh fruits and vegetables are:
Apples, 1–1.5%
Apricots, 1%
Cherries, 0.4%
Oranges, 0.5–3.5%
Carrots 1.4%
Citrus peels, 30%
Rose hips, 15%
The main raw materials for pectin production are dried citrus peels or apple pomace, both by-products of juice production. Pomace from sugar beets is also used to a small extent.
From these materials, pectin is extracted by adding hot dilute acid at pH values from 1.5 to 3.5. During several hours of extraction, the protopectin loses some of its branching and chain length and goes into solution. After filtering, the extract is concentrated in a vacuum and the pectin is then precipitated by adding ethanol or isopropanol. An old technique of precipitating pectin with aluminium salts is no longer used (apart from alcohols and polyvalent cations, pectin also precipitates with proteins and detergents).
Alcohol-precipitated pectin is then separated, washed, and dried. Treating the initial pectin with dilute acid leads to low-esterified pectins. When this process includes ammonium hydroxide (NH3(aq)), amidated pectins are obtained. After drying and milling, pectin is usually standardised with sugar, and sometimes calcium salts or organic acids, to optimise performance in a particular application.
Uses
The main use for pectin is as a gelling agent, thickening agent and stabiliser in food.
In some countries, pectin is also available as a solution or an extract, or as a blended powder, for home jam making.
The classical application is giving the jelly-like consistency to jams or marmalades, which would otherwise be sweet juices. Pectin also reduces syneresis in jams and marmalades and increases the gel strength of low-calorie jams. For household use, pectin is an ingredient in gelling sugar (also known as "jam sugar") where it is diluted to the right concentration with sugar and some citric acid to adjust pH. | Pectin | Wikipedia | 478 | 159750 | https://en.wikipedia.org/wiki/Pectin | Biology and health sciences | Carbohydrates | Biology |
For various food applications, different kinds of pectins can be distinguished by their properties, such as acidity, degree of esterification, relative number of methoxyl groups in the molecules, etc. For instance, the term "high methoxyl" refers to pectins that have a large proportion of the carboxyl groups in the pectin molecule that are esterified with methanol, compared to low methoxyl pectins:
high methoxyl pectins are defined as those with a degree of esterification equal to or above 50, are typically used in traditional jam and jelly making; such pectins require high sugar concentrations and acidic conditions to form gels, and provide a smooth texture and suitable to be used in bakery fillings and confectionery applications;
low methoxyl pectins have a degree of esterification of less than 50, can be either amidated or non-amidated: the percentage level of substitution of the amide group, defined as the degree of amidation, defines the efficacy of a pectin; low methoxyl pectins can provide a range of textures and rheological properties, depending on the calcium concentration and the calcium reactivity of the pectin chosen—amidated low methoxyl pectins are generally thermoreversible, meaning they can form gels that can melt and reform, whereas non-amidated low methoxyl pectins can form thermostable gels that withstand high temperatures; these properties make low methoxyl pectins suitable for low sugar and sugar-free applications, dairy products, and stabilizing acidic protein drinks.
For conventional jams and marmalades that contain above 60% sugar and soluble fruit solids, high-ester (high methoxyl) pectins are used. With low-ester (low methoxyl) pectins and amidated pectins, less sugar is needed, so that diet products can be made. Water extract of aiyu seeds is traditionally used in Taiwan to make aiyu jelly, where the extract gels without heating due to low-ester pectins from the seeds and the bivalent cations from the water. | Pectin | Wikipedia | 470 | 159750 | https://en.wikipedia.org/wiki/Pectin | Biology and health sciences | Carbohydrates | Biology |
Pectin is used in confectionery jellies to give a good gel structure, a clean bite and to confer a good flavour release. Pectin can also be used to stabilise acidic protein drinks, such as drinking yogurt, to improve the mouth-feel and the pulp stability in juice based drinks and as a fat substitute in baked goods.
Typical levels of pectin used as a food additive are between 0.5 and 1.0% – this is about the same amount of pectin as in fresh fruit.
In medicine, pectin increases viscosity and volume of stool so that it is used against constipation and diarrhea. Until 2002, it was one of the main ingredients used in Kaopectate – a medication to combat diarrhea – along with kaolinite. It has been used in gentle heavy metal removal from biological systems. Pectin is also used in throat lozenges as a demulcent.
In cosmetic products, pectin acts as a stabiliser. Pectin is also used in wound healing preparations and speciality medical adhesives, such as colostomy devices.
Sriamornsak revealed that pectin could be used in various oral drug delivery platforms, e.g., controlled release systems, gastro-retentive systems, colon-specific delivery systems and mucoadhesive delivery systems, according to its intoxicity and low cost. It was found that pectin from different sources provides different gelling abilities, due to variations in molecular size and chemical composition. Like other natural polymers, a major problem with pectin is inconsistency in reproducibility between samples, which may result in poor reproducibility in drug delivery characteristics.
In ruminant nutrition, depending on the extent of lignification of the cell wall, pectin is up to 90% digestible by bacterial enzymes. Ruminant nutritionists recommend that the digestibility and energy concentration in forages be improved by increasing pectin concentration in the forage.
In cigars, pectin is considered an excellent substitute for vegetable glue and many cigar smokers and collectors use pectin for repairing damaged tobacco leaves on their cigars. | Pectin | Wikipedia | 466 | 159750 | https://en.wikipedia.org/wiki/Pectin | Biology and health sciences | Carbohydrates | Biology |
Yablokov et al., writing in Chernobyl: Consequences of the Catastrophe for People and the Environment, quote research conducted by the Ukrainian Center of Radiation Medicine and the Belarusian Institute of Radiation Medicine and Endocrinology, concluded, regarding pectin's radioprotective effects, that "adding pectin preparations to the food of inhabitants of the Chernobyl-contaminated regions promotes an effective excretion of incorporated radionuclides" such as cesium-137. The authors reported on the positive results of using pectin food additive preparations in a number of clinical studies conducted on children in severely polluted areas, with up to 50% improvement over control groups.
During the Second World War, Allied pilots were provided with maps printed on silk, for navigation in escape and evasion efforts. The printing process at first proved nearly impossible because the several layers of ink immediately ran, blurring outlines and rendering place names illegible until the inventor of the maps, Clayton Hutton, mixed a little pectin with the ink and at once the pectin coagulated the ink and prevented it from running, allowing small topographic features to be clearly visible.
Legal status
At the Joint FAO/WHO Expert Committee Report on Food Additives and in the European Union, no numerical acceptable daily intake (ADI) has been set, as pectin is considered safe.
The European Union (EU) has not set a daily intake limit for two types of pectin, known as E440(i) and Amidated Pectin E440(ii). The EU has established purity standards for these additives in the EU Commission Regulation (EU)/231/2012. Pectin can be used as needed in most food categories, a concept referred to as "quantum satis". The European Food Safety Authority (EFSA) conducted a re-evaluation of Pectin E440(i) and Amidated Pectin E440(ii) in 2017. The EFSA concluded that the use of these food additives poses no safety concern for the general population. Furthermore, the agency stated that it is not necessary to establish a numerical value for the Acceptable Daily Intake (ADI).
In the United States, pectin is generally recognised as safe for human consumption. | Pectin | Wikipedia | 474 | 159750 | https://en.wikipedia.org/wiki/Pectin | Biology and health sciences | Carbohydrates | Biology |
In the International Numbering System (INS), pectin has the number 440. In Europe, pectins are differentiated into the E numbers E440(i) for non-amidated pectins and E440(ii) for amidated pectins. There are specifications in all national and international legislation defining its quality and regulating its use.
History
Pectin was first isolated and described in 1825 by Henri Braconnot, though the action of pectin to make jams and marmalades was known long before. To obtain well-set jams from fruits that had little or only poor quality pectin, pectin-rich fruits or their extracts were mixed into the recipe.
During the Industrial Revolution, the makers of fruit preserves turned to producers of apple juice to obtain dried apple pomace that was cooked to extract pectin. Later, in the 1920s and 1930s, factories were built that commercially extracted pectin from dried apple pomace, and later citrus peel, in regions that produced apple juice in both the US and Europe.
Pectin was first sold as a liquid extract, but is now most often used as dried powder, which is easier than a liquid to store and handle. | Pectin | Wikipedia | 252 | 159750 | https://en.wikipedia.org/wiki/Pectin | Biology and health sciences | Carbohydrates | Biology |
In mathematical optimization, the method of Lagrange multipliers is a strategy for finding the local maxima and minima of a function subject to equation constraints (i.e., subject to the condition that one or more equations have to be satisfied exactly by the chosen values of the variables). It is named after the mathematician Joseph-Louis Lagrange.
Summary and rationale
The basic idea is to convert a constrained problem into a form such that the derivative test of an unconstrained problem can still be applied. The relationship between the gradient of the function and gradients of the constraints rather naturally leads to a reformulation of the original problem, known as the Lagrangian function or Lagrangian. In the general case, the Lagrangian is defined as
for functions ; the notation denotes an inner product. The value is called the Lagrange multiplier.
In simple cases, where the inner product is defined as the dot product, the Lagrangian is
The method can be summarized as follows: in order to find the maximum or minimum of a function subject to the equality constraint , find the stationary points of considered as a function of and the Lagrange multiplier . This means that all partial derivatives should be zero, including the partial derivative with respect to .
or equivalently
The solution corresponding to the original constrained optimization is always a saddle point of the Lagrangian function, which can be identified among the stationary points from the definiteness of the bordered Hessian matrix.
The great advantage of this method is that it allows the optimization to be solved without explicit parameterization in terms of the constraints. As a result, the method of Lagrange multipliers is widely used to solve challenging constrained optimization problems. Further, the method of Lagrange multipliers is generalized by the Karush–Kuhn–Tucker conditions, which can also take into account inequality constraints of the form for a given constant .
Statement
The following is known as the Lagrange multiplier theorem.
Let be the objective function and let be the constraints function, both belonging to (that is, having continuous first derivatives). Let be an optimal solution to the following optimization problem such that, for the matrix of partial derivatives , : | Lagrange multiplier | Wikipedia | 458 | 159974 | https://en.wikipedia.org/wiki/Lagrange%20multiplier | Mathematics | Multivariable and vector calculus | null |
Then there exists a unique Lagrange multiplier such that (Note that this is a somewhat conventional thing where is clearly treated as a column vector to ensure that the dimensions match. But, we might as well make it just a row vector without taking the transpose.)
The Lagrange multiplier theorem states that at any local maximum (or minimum) of the function evaluated under the equality constraints, if constraint qualification applies (explained below), then the gradient of the function (at that point) can be expressed as a linear combination of the gradients of the constraints (at that point), with the Lagrange multipliers acting as coefficients. This is equivalent to saying that any direction perpendicular to all gradients of the constraints is also perpendicular to the gradient of the function. Or still, saying that the directional derivative of the function is in every feasible direction.
Single constraint
For the case of only one constraint and only two choice variables (as exemplified in Figure 1), consider the optimization problem
(Sometimes an additive constant is shown separately rather than being included in , in which case the constraint is written as in Figure 1.) We assume that both and have continuous first partial derivatives. We introduce a new variable () called a Lagrange multiplier (or Lagrange undetermined multiplier) and study the Lagrange function (or Lagrangian or Lagrangian expression) defined by
where the term may be either added or subtracted. If is a maximum of for the original constrained problem and then there exists such that () is a stationary point for the Lagrange function (stationary points are those points where the first partial derivatives of are zero). The assumption is called constraint qualification. However, not all stationary points yield a solution of the original problem, as the method of Lagrange multipliers yields only a necessary condition for optimality in constrained problems. Sufficient conditions for a minimum or maximum also exist, but if a particular candidate solution satisfies the sufficient conditions, it is only guaranteed that that solution is the best one locally – that is, it is better than any permissible nearby points. The global optimum can be found by comparing the values of the original objective function at the points satisfying the necessary and locally sufficient conditions. | Lagrange multiplier | Wikipedia | 465 | 159974 | https://en.wikipedia.org/wiki/Lagrange%20multiplier | Mathematics | Multivariable and vector calculus | null |
The method of Lagrange multipliers relies on the intuition that at a maximum, cannot be increasing in the direction of any such neighboring point that also has . If it were, we could walk along to get higher, meaning that the starting point wasn't actually the maximum. Viewed in this way, it is an exact analogue to testing if the derivative of an unconstrained function is , that is, we are verifying that the directional derivative is 0 in any relevant (viable) direction.
We can visualize contours of given by for various values of , and the contour of given by .
Suppose we walk along the contour line with We are interested in finding points where almost does not change as we walk, since these points might be maxima.
There are two ways this could happen:
We could touch a contour line of , since by definition does not change as we walk along its contour lines. This would mean that the tangents to the contour lines of and are parallel here.
We have reached a "level" part of , meaning that does not change in any direction.
To check the first possibility (we touch a contour line of ), notice that since the gradient of a function is perpendicular to the contour lines, the tangents to the contour lines of and are parallel if and only if the gradients of and are parallel. Thus we want points where and
for some
where
are the respective gradients. The constant is required because although the two gradient vectors are parallel, the magnitudes of the gradient vectors are generally not equal. This constant is called the Lagrange multiplier. (In some conventions is preceded by a minus sign).
Notice that this method also solves the second possibility, that is level: if is level, then its gradient is zero, and setting is a solution regardless of .
To incorporate these conditions into one equation, we introduce an auxiliary function
and solve
Note that this amounts to solving three equations in three unknowns. This is the method of Lagrange multipliers.
Note that implies as the partial derivative of with respect to is
To summarize
The method generalizes readily to functions on variables
which amounts to solving equations in unknowns.
The constrained extrema of are critical points of the Lagrangian , but they are not necessarily local extrema of (see below). | Lagrange multiplier | Wikipedia | 483 | 159974 | https://en.wikipedia.org/wiki/Lagrange%20multiplier | Mathematics | Multivariable and vector calculus | null |
One may reformulate the Lagrangian as a Hamiltonian, in which case the solutions are local minima for the Hamiltonian. This is done in optimal control theory, in the form of Pontryagin's minimum principle.
The fact that solutions of the method of Lagrange multipliers are not necessarily extrema of the Lagrangian, also poses difficulties for numerical optimization. This can be addressed by minimizing the magnitude of the gradient of the Lagrangian, as these minima are the same as the zeros of the magnitude, as illustrated in Example 5: Numerical optimization.
Multiple constraints
The method of Lagrange multipliers can be extended to solve problems with multiple constraints using a similar argument. Consider a paraboloid subject to two line constraints that intersect at a single point. As the only feasible solution, this point is obviously a constrained extremum. However, the level set of is clearly not parallel to either constraint at the intersection point (see Figure 3); instead, it is a linear combination of the two constraints' gradients. In the case of multiple constraints, that will be what we seek in general: The method of Lagrange seeks points not at which the gradient of is a multiple of any single constraint's gradient necessarily, but in which it is a linear combination of all the constraints' gradients.
Concretely, suppose we have constraints and are walking along the set of points satisfying Every point on the contour of a given constraint function has a space of allowable directions: the space of vectors perpendicular to The set of directions that are allowed by all constraints is thus the space of directions perpendicular to all of the constraints' gradients. Denote this space of allowable moves by and denote the span of the constraints' gradients by Then the space of vectors perpendicular to every element of
We are still interested in finding points where does not change as we walk, since these points might be (constrained) extrema. We therefore seek such that any allowable direction of movement away from is perpendicular to (otherwise we could increase by moving along that allowable direction). In other words, Thus there are scalars such that
These scalars are the Lagrange multipliers. We now have of them, one for every constraint.
As before, we introduce an auxiliary function
and solve
which amounts to solving equations in unknowns.
The constraint qualification assumption when there are multiple constraints is that the constraint gradients at the relevant point are linearly independent. | Lagrange multiplier | Wikipedia | 510 | 159974 | https://en.wikipedia.org/wiki/Lagrange%20multiplier | Mathematics | Multivariable and vector calculus | null |
Modern formulation via differentiable manifolds
The problem of finding the local maxima and minima subject to constraints can be generalized to finding local maxima and minima on a differentiable manifold In what follows, it is not necessary that be a Euclidean space, or even a Riemannian manifold. All appearances of the gradient (which depends on a choice of Riemannian metric) can be replaced with the exterior derivative
Single constraint
Let be a smooth manifold of dimension Suppose that we wish to find the stationary points of a smooth function when restricted to the submanifold defined by where is a smooth function for which is a regular value.
Let and be the exterior derivatives of and . Stationarity for the restriction at means Equivalently, the kernel contains In other words, and are proportional 1-forms. For this it is necessary and sufficient that the following system of equations holds:
where denotes the exterior product. The stationary points are the solutions of the above system of equations plus the constraint Note that the equations are not independent, since the left-hand side of the equation belongs to the subvariety of consisting of decomposable elements.
In this formulation, it is not necessary to explicitly find the Lagrange multiplier, a number such that
Multiple constraints
Let and be as in the above section regarding the case of a single constraint. Rather than the function described there, now consider a smooth function with component functions for which is a regular value. Let be the submanifold of defined by
is a stationary point of if and only if contains For convenience let and where denotes the tangent map or Jacobian ( can be canonically identified with ). The subspace has dimension smaller than that of , namely and belongs to if and only if belongs to the image of Computationally speaking, the condition is that belongs to the row space of the matrix of or equivalently the column space of the matrix of (the transpose). If denotes the exterior product of the columns of the matrix of the stationary condition for at becomes
Once again, in this formulation it is not necessary to explicitly find the Lagrange multipliers, the numbers such that
Interpretation of the Lagrange multipliers
In this section, we modify the constraint equations from the form to the form where the are real constants that are considered to be additional arguments of the Lagrangian expression . | Lagrange multiplier | Wikipedia | 476 | 159974 | https://en.wikipedia.org/wiki/Lagrange%20multiplier | Mathematics | Multivariable and vector calculus | null |
Often the Lagrange multipliers have an interpretation as some quantity of interest. For example, by parametrising the constraint's contour line, that is, if the Lagrangian expression is
then
So, is the rate of change of the quantity being optimized as a function of the constraint parameter.
As examples, in Lagrangian mechanics the equations of motion are derived by finding stationary points of the action, the time integral of the difference between kinetic and potential energy. Thus, the force on a particle due to a scalar potential, , can be interpreted as a Lagrange multiplier determining the change in action (transfer of potential to kinetic energy) following a variation in the particle's constrained trajectory.
In control theory this is formulated instead as costate equations.
Moreover, by the envelope theorem the optimal value of a Lagrange multiplier has an interpretation as the marginal effect of the corresponding constraint constant upon the optimal attainable value of the original objective function: If we denote values at the optimum with a star (), then it can be shown that
For example, in economics the optimal profit to a player is calculated subject to a constrained space of actions, where a Lagrange multiplier is the change in the optimal value of the objective function (profit) due to the relaxation of a given constraint (e.g. through a change in income); in such a context is the marginal cost of the constraint, and is referred to as the shadow price.
Sufficient conditions
Sufficient conditions for a constrained local maximum or minimum can be stated in terms of a sequence of principal minors (determinants of upper-left-justified sub-matrices) of the bordered Hessian matrix of second derivatives of the Lagrangian expression.
Examples
Example 1
Suppose we wish to maximize subject to the constraint The feasible set is the unit circle, and the level sets of are diagonal lines (with slope −1), so we can see graphically that the maximum occurs at and that the minimum occurs at
For the method of Lagrange multipliers, the constraint is
hence the Lagrangian function,
is a function that is equivalent to when is set to .
Now we can calculate the gradient:
and therefore:
Notice that the last equation is the original constraint.
The first two equations yield
By substituting into the last equation we have:
so
which implies that the stationary points of are
Evaluating the objective function at these points yields
Thus the constrained maximum is and the constrained minimum is . | Lagrange multiplier | Wikipedia | 511 | 159974 | https://en.wikipedia.org/wiki/Lagrange%20multiplier | Mathematics | Multivariable and vector calculus | null |
Example 2
Now we modify the objective function of Example 1 so that we minimize instead of again along the circle Now the level sets of are still lines of slope −1, and the points on the circle tangent to these level sets are again and These tangency points are maxima of
On the other hand, the minima occur on the level set for (since by its construction cannot take negative values), at and where the level curves of are not tangent to the constraint. The condition that correctly identifies all four points as extrema; the minima are characterized in by and the maxima by
Example 3
This example deals with more strenuous calculations, but it is still a single constraint problem.
Suppose one wants to find the maximum values of
with the condition that the - and -coordinates lie on the circle around the origin with radius That is, subject to the constraint
As there is just a single constraint, there is a single multiplier, say
The constraint is identically zero on the circle of radius Any multiple of may be added to leaving unchanged in the region of interest (on the circle where our original constraint is satisfied).
Applying the ordinary Lagrange multiplier method yields
from which the gradient can be calculated:
And therefore:
(iii) is just the original constraint. (i) implies or If then by (iii) and consequently from (ii). If substituting this into (ii) yields Substituting this into (iii) and solving for gives Thus there are six critical points of
Evaluating the objective at these points, one finds that
Therefore, the objective function attains the global maximum (subject to the constraints) at and the global minimum at The point is a local minimum of and is a local maximum of as may be determined by consideration of the Hessian matrix of
Note that while is a critical point of it is not a local extremum of We have
Given any neighbourhood of one can choose a small positive and a small of either sign to get values both greater and less than This can also be seen from the Hessian matrix of evaluated at this point (or indeed at any of the critical points) which is an indefinite matrix. Each of the critical points of is a saddle point of
Example 4 – Entropy
Suppose we wish to find the discrete probability distribution on the points with maximal information entropy. This is the same as saying that we wish to find the least structured probability distribution on the points In other words, we wish to maximize the Shannon entropy equation: | Lagrange multiplier | Wikipedia | 500 | 159974 | https://en.wikipedia.org/wiki/Lagrange%20multiplier | Mathematics | Multivariable and vector calculus | null |
For this to be a probability distribution the sum of the probabilities at each point must equal 1, so our constraint is:
We use Lagrange multipliers to find the point of maximum entropy, across all discrete probability distributions on We require that:
which gives a system of equations, such that:
Carrying out the differentiation of these equations, we get
This shows that all are equal (because they depend on only). By using the constraint
we find
Hence, the uniform distribution is the distribution with the greatest entropy, among distributions on points.
Example 5 – Numerical optimization
The critical points of Lagrangians occur at saddle points, rather than at local maxima (or minima). Unfortunately, many numerical optimization techniques, such as hill climbing, gradient descent, some of the quasi-Newton methods, among others, are designed to find local maxima (or minima) and not saddle points. For this reason, one must either modify the formulation to ensure that it's a minimization problem (for example, by extremizing the square of the gradient of the Lagrangian as below), or else use an optimization technique that finds stationary points (such as Newton's method without an extremum seeking line search) and not necessarily extrema.
As a simple example, consider the problem of finding the value of that minimizes constrained such that (This problem is somewhat untypical because there are only two values that satisfy this constraint, but it is useful for illustration purposes because the corresponding unconstrained function can be visualized in three dimensions.)
Using Lagrange multipliers, this problem can be converted into an unconstrained optimization problem:
The two critical points occur at saddle points where and .
In order to solve this problem with a numerical optimization technique, we must first transform this problem such that the critical points occur at local minima. This is done by computing the magnitude of the gradient of the unconstrained optimization problem.
First, we compute the partial derivative of the unconstrained problem with respect to each variable:
If the target function is not easily differentiable, the differential with respect to each variable can be approximated as
where is a small value.
Next, we compute the magnitude of the gradient, which is the square root of the sum of the squares of the partial derivatives: | Lagrange multiplier | Wikipedia | 477 | 159974 | https://en.wikipedia.org/wiki/Lagrange%20multiplier | Mathematics | Multivariable and vector calculus | null |
(Since magnitude is always non-negative, optimizing over the squared-magnitude is equivalent to optimizing over the magnitude. Thus, the "square root" may be omitted from these equations with no expected difference in the results of optimization.)
The critical points of occur at and , just as in Unlike the critical points in however, the critical points in occur at local minima, so numerical optimization techniques can be used to find them.
Applications
Control theory
In optimal control theory, the Lagrange multipliers are interpreted as costate variables, and Lagrange multipliers are reformulated as the minimization of the Hamiltonian, in Pontryagin's minimum principle.
Nonlinear programming
The Lagrange multiplier method has several generalizations. In nonlinear programming there are several multiplier rules, e.g. the Carathéodory–John Multiplier Rule and the Convex Multiplier Rule, for inequality constraints.
Economics
In many models in mathematical economics such as general equilibrium models, consumer behavior is implemented as utility maximization and firm behavior as profit maximization, both entities being subject to constraints such as budget constraints and production constraints. The usual way to determine an optimal solution is achieved by maximizing some function, where the constraints are enforced using Lagrangian multipliers.
Power systems
Methods based on Lagrange multipliers have applications in power systems, e.g. in distributed-energy-resources (DER) placement and load shedding.
Safe Reinforcement Learning
The method of Lagrange multipliers applies to constrained Markov decision processes.
It naturally produces gradient-based primal-dual algorithms in safe reinforcement learning.
Normalized solutions
Considering the PDE problems with constraints, i.e., the study of the properties of the normalized solutions, Lagrange multipliers play an important role. | Lagrange multiplier | Wikipedia | 376 | 159974 | https://en.wikipedia.org/wiki/Lagrange%20multiplier | Mathematics | Multivariable and vector calculus | null |
Tsetse ( , or ) (sometimes spelled tzetze; also known as tik-tik flies) are large, biting flies that inhabit much of tropical Africa. Tsetse flies include all the species in the genus Glossina, which are placed in their own family, Glossinidae. The tsetse is an obligate parasite, which lives by feeding on the blood of vertebrate animals. Tsetse has been extensively studied because of their role in transmitting disease. They have pronounced economic and public health impacts in sub-Saharan Africa as the biological vectors of trypanosomes, causing human and animal trypanosomiasis.
Tsetse can be distinguished from other large flies by two easily-observed features: primarily, tsetse fold their wings over their abdomens completely when they are resting (so that one wing rests directly on top of the other); Secondly, tsetse also have a long proboscis, extending directly forward, which is attached by a distinct bulb to the bottom of their heads.
Fossilized tsetse has been recovered from Paleogene-aged rocks in the United States and Germany. Twenty-three extant species of tsetse flies are known from the African continent as well as the Arabian Peninsula.
Terminology
Tsetse without the "fly" has become more common in English, particularly in the scientific and development communities.
The word is pronounced tseh-tseh in the Sotho languages and is easily rendered in other African languages. During World War II, a British de Havilland antisubmarine aircraft was known as the Tsetse Mosquito.
Biology
The biology of tsetse is relatively well understood by entomologists. They have been extensively studied because of their medical, veterinary, and economic importance, because the flies can be raised in a laboratory, and because they are relatively large, facilitating their analysis.
Morphology
Tsetse flies can be seen as independent individuals in three forms: as third-instar larvae, pupae, and adults.
Tsetse first becomes separate from their mothers during the third larval instar, during which they have the typical appearance of maggots. However, this life stage is short, lasting at most a few hours, and is almost never observed outside of the laboratory. | Tsetse fly | Wikipedia | 465 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Tsetse next develops a hard external case, the puparium, and become pupae - small, hard-shelled oblongs with two distinctively small, dark lobes at the tail (breathing) end. Tsetse pupae are under long. Within the puparial shell, tsetse complete the last two larval instars and the pupal stage.
At the end of the pupal stage, tsetse emerges as adult flies. The adults are relatively large flies, with lengths of , and have a recognizable shape, or bauplan, which makes them easy to distinguish from other flies. Tsetse have large heads, distinctly separated eyes, and unusual antennae. The thorax is quite large, while the abdomen is wider, rather than elongated, and shorter than the wings.
Four characteristics collectively separate adult tsetse from other kinds of flies:
Anatomy
Like all other insects, tsetse flies have an adult body comprising three visibly distinct parts: the head, the thorax, and the abdomen.
The head has large eyes, distinctly separated on each side, and a distinct, forward-pointing proboscis attached underneath by a large bulb. The thorax is large, made of three fused segments. Three pairs of legs are attached to the thorax, as are two wings and two halteres. The abdomen is short but wide and changes dramatically in volume during feeding.
The internal anatomy of the tsetse is fairly typical of the insects; the crop is large enough to accommodate a huge increase in size during feeding, as tsetse can take a blood meal equal in weight to themselves. The dipteran crop is heavily understudied, with Glossina being one of the few genera having relatively reliable information available: Moloo and Kutuza 1970 for G. brevipalpis (including its innervation) and Langley 1965 for G. morsitans. The reproductive tract of adult females includes a uterus, which can become large enough to hold the third-instar larva at the end of each pregnancy.
Most tsetse flies are, physically, very tough. Houseflies, and even horseflies, are easily killed with a flyswatter, for example; a great deal of effort is needed to crush a tsetse fly.
Life cycle | Tsetse fly | Wikipedia | 470 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Tsetse has an unusual life cycle, which may be due to the richness of their blood food source. A female fertilizes only one egg at a time; she will retain each egg within her uterus, the offspring developing internally (during the first three larval stages), in an adaptation called adenotrophic viviparity. During this time, the female feeds the developing offspring with a milky substance (secreted by a modified gland) in the uterus.
In the third larval stage, the tsetse larvae leave the uterus and begin an independent life. The newly-birthed larvae crawl into the ground and develop a hard outer shell (called the puparial case), within which they complete their morphological transformations into adult flies.
The larval life stage has a variable duration, generally 20 to 30 days, and the larvae must rely on stored resources during this time. The importance of the richness and quality of blood to this stage can be seen; all tsetse development (prior to emerging from the puparial case as a full adult) occurs without feeding, with only the nutrition provided by the mother fly. She must get enough energy for her own survival (in addition to the needs of her developing offspring), as well as for the stored resources that her offspring will require until they emerge as adults.
Technically, these insects undergo the standard development process of insects, beginning with oocyte formation, ovulation, fertilization, and development of the egg; following egg development and birth is the three larval stages, a pupal stage, and the emergence and maturation of the adult.
Hosts
Overall Suidae are the most important hosts. Waterbuck (Kobus ellipsiprymnus) are unmolested by Glossina because they produce volatiles which act as repellents. Waterbuck odor volatiles are under testing and development as repellents to protect livestock. By species, bloodmeals are derived from:
Genetics
The genome of Glossina morsitans was sequenced in 2014.
Symbionts
Tsetse flies have at least three bacterial symbionts. The primary symbiont is Wigglesworthia (Wigglesworthia glossinidia), which live within the fly's bacteriocytes. The second symbiont is Sodalis (Sodalis glossinidius) intercellularly or intracellularly, and the third is some kind of Wolbachia. | Tsetse fly | Wikipedia | 509 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Diseases
The salivary gland hypertrophy virus causes abnormal bleeding in the lobes of the crop of G. m. centralis and G. m. morsitans.
Systematics
Tsetse flies are members of the order Diptera, the true flies. They belong to the superfamily Hippoboscoidea, in which the tsetse's family, the Glossinidae, is one of four families of blood-feeding obligate parasites.
Up to 34 species and subspecies of tsetse flies are recognized, depending on the particular classification used.
Current classifications place all species of tsetse fly in a single genus named Glossina, with most considering the genus as the sole member of the family Glossinidae.
Species
The tsetse genus is generally split into three groups of species based on a combination of distributional, ecological, behavioral, molecular and morphological characteristics. The genus includes; savannah flies, forest flies and riverine and lacustrine flies.
Savannah flies
The 'savannah' flies: (Morsitans group, subgenus Glossina s.s.):
Glossina austeni (Newstead, 1912) patr. of Austen
Glossina longipalpis (Wiedemann, 1830)
Glossina morsitans (Westwood, 1851)
Glossina morsitans morsitans (Westwood, 1850)
Glossina morsitans submorsitans
Glossina morsitans centralis (Machado, 1970)
Glossina pallidipes (Austen, 1903)
Glossina swynnertoni (Austen, 1923) patr. of Swynnerton
Forest flies
The 'forest' flies: (Fusca group, subgenus Austenina): | Tsetse fly | Wikipedia | 342 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Glossina brevipalpis (Newstead, 1910)
Glossina fusca (Walker, 1849)
Glossina fusca fusca (Walker, 1849)
Glossina fusca congolensis (Newstead and Evans, 1921)
Glossina fuscipleuris (Austen, 1911)
Glossina frezili (Gouteux, 1987)
Glossina haningtoni (Newstead and Evans, 1922)
Glossina longipennis (Corti, 1895)
Glossina medicorum (Austen, 1911)
Glossina nashi (Potts, 1955)
Glossina nigrofusca (Newstead, 1911)
Glossina nigrofusca nigrofusca (Newstead, 1911)
Glossina nigrofusca hopkinsi (van Emden, 1944)
Glossina severini (Newstead, 1913)
Glossina schwetzi (Newstead and Evans, 1921)
Glossina severini (Newstead, 1913)
Glossina tabaniformis (Westwood, 1850)
Glossina vanhoofi (Henrard, 1952)
Riverine and lacustrine flies
The 'riverine' and 'lacustrine' flies: (Palpalis group, subgenus Nemorhina):
Glossina caliginea (Austen, 1911)
Glossina fuscipes (Newstead, 1911)
Glossina fuscipes fuscipes (Newstead, 1911)
Glossina fuscipes martinii (Zumpt, 1935)
Glossina fuscipes quanzensis (Pires, 1948)
Glossina pallicera (Bigot, 1891)
Glossina pallicera pallicera (Bigot, 1891)
Glossina pallicera newsteadi (Austen, 1929) patr. of Newstead
Glossina palpalis (Robineau-Desvoidy, 1830)
Glossina palpalis palpalis (Robineau-Desvoidy, 1830)
Glossina palpalis gambiensis (Vanderplank, 1911)
Glossina tachinoides (Westwood, 1850)
Evolutionary history
Fossil glossinids are known from the Florissant Formation in North America and the Enspel Lagerstätte of Germany, dating to the late Eocene and late Oligocene respectively. | Tsetse fly | Wikipedia | 468 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Range
Glossina is almost entirely restricted to wooded grasslands and forested areas of the Afrotropics. As of 1990, tsetse flies were reported from a maximum latitude of approximately 15° north in Senegal (Niayes Region), to a minimum of 28.5° south in South Africa (KwaZulu-Natal Province).
Only two subspecies - G. f. fuscipes and G. m. submorsitans - are present in the very southwest of Saudi Arabia. Although Carter found G. tachiniodes in 1903 nearby, near Aden in southern Yemen, there have been no confirmations since.
Trypanosomiasis
Tsetse are biological vectors of trypanosomes, meaning that in the process of feeding, they acquire and then transmit small, single-celled trypanosomes from infected vertebrate hosts to uninfected animals. Some tsetse-transmitted trypanosome species cause trypanosomiasis, an infectious disease. In humans, tsetse transmitted trypanosomiasis is called sleeping sickness. In animals, tsetse-vectored trypanosomiases include nagana, souma (a French term which may not be a distinct condition), and surra according to the animal infected and the trypanosome species involved. The usage is not strict and while nagana generally refers to the disease in cattle and horses it is commonly used for any of the animal trypanosomiases.
Trypanosomes are animal parasites, specifically protozoans of the genus Trypanosoma. These organisms are about the size of red blood cells. Different species of trypanosomes infect different hosts. They range widely in their effects on the vertebrate hosts. Some species, such as T. theileri, do not seem to cause any health problems except perhaps in animals that are already sick.
Some strains are much more virulent. Infected flies have an altered salivary composition which lowers feeding efficiency and consequently increases the feeding time, promoting trypanosome transmission to the vertebrate host. These trypanosomes are highly evolved and have developed a life cycle that requires periods in both the vertebrate and tsetse hosts. | Tsetse fly | Wikipedia | 457 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Tsetse transmit trypanosomes in two ways, mechanical and biological transmission.
Mechanical transmission involves the direct transmission of the same individual trypanosomes taken from an infected host into an uninfected host. The name 'mechanical' reflects the similarity of this mode of transmission to mechanical injection with a syringe. Mechanical transmission requires the tsetse to feed on an infected host and acquire trypanosomes in the blood meal, and then, within a relatively short period, to feed on an uninfected host and regurgitate some of the infected blood from the first blood meal into the tissue of the uninfected animal. This type of transmission occurs most frequently when tsetse are interrupted during a blood meal and attempt to satiate themselves with another meal. Other flies, such as horse-flies, can also cause mechanical transmission of trypanosomes.
Biological transmission requires a period of incubation of the trypanosomes within the tsetse host. The term 'biological' is used because trypanosomes must reproduce through several generations inside the tsetse host during the period of incubation (development within the fly is known as the extrinsic incubation period), which requires extreme adaptation of the trypanosomes to their tsetse host. In this mode of transmission, trypanosomes reproduce through several generations, changing in morphology at certain periods. This mode of transmission also includes the sexual phase of the trypanosomes. Tsetse are believed to be more likely to become infected by trypanosomes during their first few blood meals. Tsetse infected by trypanosomes are thought to remain infected for the remainder of their lives. Because of the adaptations required for biological transmission, trypanosomes that can be transmitted biologically by tsetse cannot be transmitted in this manner by other insects.
The relative importance of these two modes of transmission for the propagation of tsetse-vectored trypanosomiases is not yet well understood. However, since the sexual phase of the trypanosome life cycle occurs within the tsetse host, biological transmission is a required step in the life cycle of the tsetse-vectored trypanosomes. | Tsetse fly | Wikipedia | 459 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
The cycle of biological transmission of trypanosomiasis involves two phases, one inside the tsetse host and the other inside the vertebrate host. Trypanosomes are not passed between a pregnant tsetse and her offspring, so all newly emerged tsetse adults are free of infection. An uninfected fly that feeds on an infected vertebrate animal may acquire trypanosomes in its proboscis or gut. These trypanosomes, depending on the species, may remain in place, move to a different part of the digestive tract, or migrate through the tsetse body into the salivary glands. When an infected tsetse bites a susceptible host, the fly may regurgitate part of a previous blood meal that contains trypanosomes, or may inject trypanosomes in its saliva. Inoculation must contain a minimum of 300 to 450 individual trypanosomes to be successful, and may contain up to 40,000 cells.
In the case of T. b. brucei infecting G. p. gambiensis, during this time the parasite changes the proteome contents of the fly's head. This may be the reason/a reason for the behavioral changes seen, especially the unnecessarily increased feeding frequency, which increases transmission opportunities. This may be due in part to the altered glucose metabolism observed, causing a perceived need for more calories. (The metabolic change, in turn, being due to complete absence of glucose-6-phosphate 1-dehydrogenase in infected flies.) Monoamine neurotransmitter synthesis is also altered: Production of aromatic L-amino acid decarboxylase - involved in dopamine and serotonin synthesis - and α-methyldopa hypersensitive protein was induced. This is very similar to the alterations in other dipteran vectors' head proteomes under infection by other eukaryotic parasites of mammals, found in another study by the same team in the same year.
The trypanosomes are injected into vertebrate muscle tissue, but make their way, first into the lymphatic system, then into the bloodstream, and eventually into the brain. The disease causes the swelling of the lymph glands, emaciation of the body, and eventually leads to death. Uninfected tsetse may bite the infected animal prior to its death and acquire the disease, thereby closing the transmission cycle. | Tsetse fly | Wikipedia | 512 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Disease hosts and vectors
The tsetse-vectored trypanosomiases affect various vertebrate species including humans, antelopes, bovine cattle, camels, horses, sheep, goats, and pigs. These diseases are caused by several different trypanosome species that may also survive in wild animals such as crocodiles and monitor lizards. The diseases have different distributions across the African continent, so are transmitted by different species. This table summarizes this information:
In humans
Human African trypanosomiasis, also called sleeping sickness, is caused by trypanosomes of the species Trypanosoma brucei. This disease is invariably fatal if left untreated, but can almost always be cured with current medicines if the disease is diagnosed early enough.
Sleeping sickness begins with a tsetse bite leading to an inoculation in the subcutaneous tissue. The infection moves into the lymphatic system, leading to a characteristic swelling of the lymph glands called Winterbottom's sign. The infection progresses into the blood stream and eventually crosses into the central nervous system and invades the brain leading to extreme lethargy and eventually to death.
The species Trypanosoma brucei, which causes the disease, has often been subdivided into three subspecies that were identified based either on the vertebrate hosts which the strain could infect or on the virulence of the disease in humans. The trypanosomes infectious to animals and not to humans were named Trypanosoma brucei brucei. Strains that infected humans were divided into two subspecies based on their different virulences: Trypanosoma brucei gambiense was thought to have a slower onset and Trypanosoma brucei rhodesiense refers to strains with a more rapid, virulent onset. This characterization has always been problematic but was the best that could be done given the knowledge of the time and the tools available for identification. A recent molecular study using restriction fragment length polymorphism analysis suggests that the three subspecies are polyphyletic, so the elucidation of the strains of T. brucei infective to humans requires a more complex explanation. Procyclins are proteins developed in the surface coating of trypanosomes whilst in their tsetse fly vector. | Tsetse fly | Wikipedia | 470 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Other forms of human trypanosomiasis also exist but are not transmitted by tsetse. The most notable is American trypanosomiasis, known as Chagas disease, which occurs in South America, caused by Trypanosoma cruzi, and transmitted by certain insects of the Reduviidae, members of the Hemiptera.
In domestic animals
Animal trypanosomiasis, also called nagana when it occurs in bovine cattle or horses or sura when it occurs in domestic pigs, is caused by several trypanosome species. These diseases reduce the growth rate, milk productivity, and strength of farm animals, generally leading to the eventual death of the infected animals. Certain species of cattle are called trypanotolerant because they can survive and grow even when infected with trypanosomes although they also have lower productivity rates when infected.
The course of the disease in animals is similar to the course of sleeping sickness in humans.
Trypanosoma congolense and Trypanosoma vivax are the two most important species infecting bovine cattle in sub-Saharan Africa. Trypanosoma simiae causes a virulent disease in swine.
Other forms of animal trypanosomiasis are also known from other areas of the globe, caused by different species of trypanosomes and transmitted without the intervention of the tsetse fly.
The tsetse fly vector ranges mostly in the central part of Africa.
Trypanosomiasis poses a considerable constraint on livestock agricultural development in tsetse fly-infested areas of sub-Saharan Africa, especially in West and Central Africa. International research conducted by ILRI in Nigeria, the Democratic Republic of the Congo and Kenya has shown that the N'Dama is the most resistant breed.
Control
The conquest of sleeping sickness and nagana would be of immense benefit to rural development and contribute to poverty alleviation and improved food security in sub-Saharan Africa. Human African trypanosomosis (HAT) and animal African trypanosomosis (AAT) are sufficiently important to make virtually any intervention against these diseases beneficial.
The disease can be managed by controlling the vector and thus reducing the incidence of the disease by disrupting the transmission cycle. Another tactic to manage the disease is to target the disease directly using surveillance and curative or prophylactic treatments to reduce the number of hosts that carry the disease. | Tsetse fly | Wikipedia | 490 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Economic analysis indicates that the cost of managing trypanosomosis through the elimination of important populations of major tsetse vectors will be covered several times by the benefits of tsetse-free status. Area-wide interventions against the tsetse and trypanosomosis problem appear more efficient and profitable if sufficiently large areas, with high numbers of cattle, can be covered.
Vector control strategies can aim at either continuous suppression or eradication of target populations. Tsetse fly eradication programmes are complex and logistically demanding activities and usually involve the integration of different control tactics, such as trypanocidal drugs, impregnated treated targets (ITT), insecticide-treated cattle (ITC), aerial spraying (Sequential Aerosol Technique - SAT) and in some situations the release of sterile males (sterile insect technique – SIT). To ensure sustainability of the results, it is critical to apply the control tactics on an area-wide basis, i.e. targeting an entire tsetse population that is preferably genetically isolated.
Control techniques
Many techniques have reduced tsetse populations, with earlier, crude methods recently replaced by methods that are cheaper, more directed, and ecologically better.
Slaughter of wild animals
One early technique involved slaughtering all the wild animals tsetse fed on. For example, the island of Principe off the west coast of Africa was entirely cleared of feral pigs in the 1930s, which led to the extirpation of the fly. While the fly eventually re-invaded in the 1950s, the new population of tsetse was free from the disease.
Land clearing
Another early technique involved complete removal of brush and woody vegetation from an area. However, the technique was not widely used and has been abandoned. Tsetse tend to rest on the trunks of trees so removing woody vegetation made the area inhospitable to the flies. Until about 1959 this was done by hand and so was quite time consuming. Glover et al 1959 describes the technique which they call "chain clearing". Chain clearing drags a chain forward between two heavy vehicles and thereby does the same job much more quickly - but still at some expense. Preventing regrowth of woody vegetation requires continuous clearing efforts which is even more expensive, and only practical where large human populations are present. Also, the clearing of woody vegetation has come to be seen as an environmental problem more than a benefit. | Tsetse fly | Wikipedia | 487 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Pesticide campaigns
Pesticides have been used to control tsetse starting initially during the early part of the twentieth century in localized efforts using the inorganic metal-based pesticides, expanding after the Second World War into massive aerial- and ground-based campaigns with organochlorine pesticides such as DDT applied as aerosol sprays at Ultra-Low Volume rates. Later, more targeted techniques used pour-on formulations in which advanced organic pesticides were applied directly to the backs of cattle.
Trapping
Tsetse populations can be monitored and effectively controlled using simple, inexpensive traps. These often use blue cloth, either in sheet or biconical form, since this color attracts the flies. The traps work by channeling the flies into a collection chamber, or by exposing the flies to insecticide sprayed on the cloth. Early traps mimicked the form of cattle, as tsetse are also attracted to large dark colors like the hides of cows and buffaloes. Some scientists put forward the idea that zebra have stripes, not as a camouflage in long grass, but because the black and white bands tend to confuse tsetse and prevent attack.
The use of chemicals as attractants to lure tsetse to the traps has been studied extensively in the late 20th century, but this has mostly been of interest to scientists rather than as an economically reasonable solution. Attractants studied have been those tsetse might use to find food, like carbon dioxide, octenol, and acetone—which are given off in animals' breath and distributed downwind in an odor plume. Synthetic versions of these chemicals can create artificial odor plumes. A cheaper approach is to place cattle urine in a half gourd near the trap. For large trapping efforts, additional traps are generally cheaper than expensive artificial attractants.
A special trapping method is applied in Ethiopia, where the BioFarm Consortium (ICIPE, BioVision Foundation, BEA, Helvetas, DLCO-EA, Praxis Ethiopia) applies the traps in a sustainable agriculture and rural development context (SARD). The traps are just the entry point, followed by improved farming, human health and marketing inputs. This method is in the final stage of testing (as of 2006). | Tsetse fly | Wikipedia | 452 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Sterile insect technique
The sterile insect technique (SIT) is a form of pest control that uses ionizing radiation (gamma ray or X-ray) to sterilize male flies that are mass-produced in special rearing facilities. The sterile males are released systematically from the ground or by air in tsetse-infested areas, where they mate with wild females, which do not produce offspring. As a result, this technique can eventually eradicate populations of wild flies. SIT is among the most environmentally friendly control tactics available, and is usually applied as the final component of an integrated campaign. It has been used to subdue the populations of many other fly species including the medfly, Ceratitis capitata.
The sustainable removal of the tsetse fly is in many cases the most cost-effective way of dealing with the T&T problem resulting in major economic benefits for subsistence farmers in rural areas. Insecticide-based methods are normally very ineffective in removing the last remnants of tsetse populations, while, on the contrary, sterile males are very effective in finding and mating the last remaining females. Therefore, the integration of the SIT as the last component of an area-wide integrated approach is essential in many situations to achieve complete eradication of the different tsetse populations, particularly in areas of more dense vegetation.
A project that was implemented from 1994 to 1997 on the Island of Unguja, Zanzibar (United Republic of Tanzania), demonstrated that, after suppression of the tsetse population with insecticides, SIT completely removed the Glossina austeni Newstead population from the Island. This was carried out without any understanding of the population genetics of G. a., but future SIT efforts can benefit from such preparation. Population genetics would help to select the Glossina population to be deployed for similarity to the target population. The eradication of the tsetse fly from Unguja Island in 1997 was followed by the disappearance of the AAT which enabled farmers to integrate livestock keeping with cropping in areas where this had been impossible before. The increased livestock and crop productivity and the possibility of using animals for transport and traction significantly contributed to an increase in the quality of people's lives. Surveys in 1999, 2002, 2014, and 2015 have confirmed this success - continued absence of tsetse and nagana on the island. | Tsetse fly | Wikipedia | 474 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
In the Niayes region of Senegal, a coastal area close to Dakar, livestock keeping was difficult due to the presence of a population of Glossina palpalis gambiensis. Feasibility studies indicated that the fly population was confined to very fragmented habitats and a population genetics study indicated that the population was genetically isolated from the main tsetse belt in the south eastern part of Senegal. After completion of the feasibility studies (2006–2010), an area-wide integrated eradication campaign that included an SIT component was started in 2011, and by 2015, the Niayes region had become almost tsetse fly free. This has allowed a change of cattle breeds from lower producing trypanotolerant breeds to higher-producing foreign breeds.
The entire target area (Block 1, 2 and 3) has a total surface of , and the first block (northern part) can be considered free of tsetse, as intensive monitoring has failed to detect since 2012 a single wild tsetse fly. The prevalence of AAT has decreased from 40 to 50% before the project started to less than 10% to date in blocks 1 and 2. Although insecticides are being used for fly suppression, they are applied for short periods on traps, nets and livestock, and are not spread into the environment. After the suppression activities are completed, no more insecticide is applied in the area. The removal of trypanosomosis will eliminate the need for constant prophylactic treatments of the cattle with trypanocidal drugs, therefore reducing residues of these drugs in the dung, meat and milk.
The main beneficiaries of the project are the many small holder farmers, the larger commercial farms and the consumers of meat and milk. According to a socio-economic survey and benefit cost analysis, after eradication of the tsetse farmers will be able to replace their local breeds with improved breeds and increase their annual income by €2.8 million. In addition, it is expected that the number of cattle will be reduced by 45%, which will result in reduced environmental impacts.
Societal impact | Tsetse fly | Wikipedia | 421 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
In the literature of environmental determinism, the tsetse has been linked to difficulties during early state formation for areas where the fly is prevalent. A 2012 study used population growth models, physiological data, and ethnographic data to examine pre-colonial agricultural practices and isolate the effects of the fly. A "tsetse suitability index" was developed from insect population growth, climate and geospatial data to simulate the fly's population steady state. An increase in the tsetse suitability index was associated with a statistically significant weakening of the agriculture, levels of urbanization, institutions and subsistence strategies. Results suggest that the tsetse decimated livestock populations, forcing early states to rely on slave labor to clear land for farming, and preventing farmers from taking advantage of natural animal fertilizers to increase crop production. These long-term effects may have kept population density low and discouraged cooperation between small-scale communities, thus preventing stronger nations from forming.
The authors also suggest that under a lower burden of tsetse, Africa would have developed differently. Agriculture (measured by the usage of large domesticated animals, intensive agriculture, plow use and female participation rate in agriculture) as well as institutions (measured by the appearance of indigenous slavery and levels of centralization) would have been more like those found in Eurasia. Qualitative support for this claim comes from archaeological findings; e.g., Great Zimbabwe is located in the African highlands where the fly does not occur, and represented the largest and technically most advanced precolonial structure in Southern sub-Sahara Africa.
Other authors are more skeptical that the tsetse fly had such an immense influence on African development. One conventional argument is that the tsetse fly made it difficult to use draught animals. Hence, wheeled forms of transportations were not used as well. While this is certainly true for areas with high densities of the fly, similar cases outside tsetse-suitable areas exist. While the fly definitely had a relevant influence on the adoption of new technologies in Africa, it has been contended that it does not represent the single root cause. | Tsetse fly | Wikipedia | 432 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
History
According to an article in the New Scientist, the depopulated and apparently primevally wild Africa seen in wildlife documentary films was formed in the 19th century by disease, a combination of rinderpest and the tsetse fly. Rinderpest is believed to have originated in Asia, later spreading through the transport of cattle. In 1887, the rinderpest virus was accidentally imported in livestock brought by an Italian expeditionary force to Eritrea. It spread rapidly, reaching Ethiopia by 1888, the Atlantic coast by 1892 and South Africa by 1897. Rinderpest, a cattle plague from central Asia, killed over 90% of the cattle of the pastoral peoples such as the Masai of east Africa. In South Africa, with no native immunity, most of the population – some 5.5 million domestic cattle – died. Pastoralists and farmers were left with no animals – their source of income – and farmers were deprived of their working animals for ploughing and irrigation. The pandemic coincided with a period of drought, causing widespread famine. The starving human populations died of smallpox, cholera, and typhoid, as well as African Sleeping Sickness and other endemic diseases. It is estimated that two-thirds of the Masai died in 1891.
The land was left emptied of its cattle and its people, enabling the colonial powers Germany and Britain to take over Tanzania and Kenya with little effort. With greatly reduced grazing, grassland turned rapidly to bush. The closely cropped grass sward was replaced in a few years by woody grassland and thornbush, ideal habitat for tsetse flies. Wild mammal populations increased rapidly, accompanied by the tsetse fly. Highland regions of east Africa which had been free of tsetse fly were colonised by the pest, accompanied by sleeping sickness, until then unknown in the area. Millions of people died of the disease in the early 20th century. | Tsetse fly | Wikipedia | 382 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
The areas occupied by the tsetse fly were largely barred to animal husbandry. Sleeping sickness was dubbed "the best game warden in Africa" by conservationists, who assumed that the land, empty of people and full of game animals, had always been like that. Julian Huxley of the World Wildlife Fund called the plains of east Africa "a surviving sector of the rich natural world as it was before the rise of modern man". They created numerous large reserves for hunting safaris. In 1909 the newly retired president Theodore Roosevelt went on a safari that brought over 10,000 animal carcasses to America. Later, much of the land was turned over to nature reserves and national parks such as the Serengeti, Masai Mara, Kruger and Okavango Delta. The result, across eastern and southern Africa, is a modern landscape of manmade ecosystems: farmland and pastoral land largely free of bush and tsetse fly; and bush controlled by the tsetse fly.
Although the colonial powers saw the disease as a threat to their interests, and acted accordingly to bring transmission almost to a halt in the 1960s, this improved situation led to a laxity of surveillance and management by the newly independent governments covering the same areas - and a resurgence that became a crisis again in the 1990s.
Current situation
Tsetse flies are regarded as a major cause of rural poverty in sub-Saharan Africa because they prevent mixed farming. The land infested with tsetse flies is often cultivated by people using hoes rather than more efficient draught animals because nagana, the disease transmitted by tsetse, weakens and often kills these animals. Cattle that do survive produce little milk, pregnant cows often abort their calves, and manure is not available to fertilize the worn-out soils. | Tsetse fly | Wikipedia | 363 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
The disease nagana or African animal trypanosomiasis (AAT) causes gradual health decline in infected livestock, reduces milk and meat production, and increases abortion rates. Animals eventually succumb to the disease - annual cattle deaths caused by trypanosomiasis are estimated at 3 million, reducing annual cattle production value by US$600m-US$1.2b. This has an enormous impact on the livelihood of farmers who live in tsetse-infested areas, as infected animals cannot be used to plough the land, and keeping cattle is only feasible when the animals are kept under constant prophylactic treatment with trypanocidal drugs, often with associated problems of drug resistance, counterfeited drugs, and suboptimal dosage. The overall annual direct lost potential in livestock and crop production was estimated at US$4.5 billion-US$4.75b.
The tsetse fly lives in nearly in sub-Saharan Africa (mostly wet tropical forest) and many parts of this large area is fertile land that is left uncultivated—a so-called green desert not used by humans and cattle. Most of the 38 countries infested with tsetse are poor, debt-ridden and underdeveloped. Of the 38 tsetse-infested countries, 32 are low-income, food-deficit countries, 29 are least developed countries, and 30 or 34 are among the 40 most heavily indebted poor countries. Eradicating the tsetse and trypanosomiasis (T&T) problem would allow rural Africans to use these areas for animal husbandry or the cultivation of crops and hence increase food production. Only 45 million cattle, of 172 million present in sub-Saharan Africa, are kept in tsetse-infested areas but are often forced into fragile ecosystems like highlands or the semiarid Sahel zone, which increases overgrazing and overuse of land for food production.
In addition to this direct impact, the presence of tsetse and trypanosomiasis discourages the use of more productive exotic and cross-bred cattle, depresses the growth and affects the distribution of livestock populations, reduces the potential opportunities for livestock and crop production (mixed farming) through less draught power to cultivate land and less manure to fertilize (in an environment-friendly way) soils for better crop production, and affects human settlements (people tend to avoid areas with tsetse flies). | Tsetse fly | Wikipedia | 511 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Tsetse flies transmit a similar disease to humans, called African trypanosomiasis, human African trypanosomiasis (HAT) or sleeping sickness. An estimated 60-70 million people in 20 countries are at different levels of risk and only 3-4 million people are covered by active surveillance. The DALY index (disability-adjusted life years), an indicator to quantify the burden of disease, includes the impact of both the duration of life lost due to premature death and the duration of life lived with a disability. The annual burden of sleeping sickness is estimated at 2 million DALYs. Since the disease tends to affect economically active adults, the total cost to a family with a patient is about 25% of a year's income.
History of study
In East Africa, C. F. M. Swynnerton played a large role in the first half of the 20th century. Swynnerton did much of the earliest tsetse ecology research. For this E. E. Austen named a patronymic taxon for him, G. swynnertoni in 1922.
Resistance to trypanosomes
Tsetse flies have an arsenal of immune defenses to resist each stage of the trypanosome infectious cycle, and thus are relatively refractory to trypanosome infection. Among the host flies' defenses is the production of hydrogen peroxide, a reactive oxygen species that damages DNA. These defenses limit the population of infected flies. | Tsetse fly | Wikipedia | 296 | 160067 | https://en.wikipedia.org/wiki/Tsetse%20fly | Biology and health sciences | Flies (Diptera) | null |
Media player software is a type of application software for playing multimedia computer files like audio and video files. Media players commonly display standard media control icons known from physical devices such as tape recorders and CD players, such as play ( ), pause ( ), fastforward (⏩️), rewind (⏪), and stop ( ) buttons. In addition, they generally have progress bars (or "playback bars"), which are sliders to locate the current position in the duration of the media file.
Mainstream operating systems have at least one default media player. For example, Windows comes with Windows Media Player, Microsoft Movies & TV and Groove Music, while macOS comes with QuickTime Player and Music. Linux distributions come with different media players, such as SMPlayer, Amarok, Audacious, Banshee, MPlayer, mpv, Rhythmbox, Totem, VLC media player, and xine. Android comes with YouTube Music for audio and Google Photos for video, and smartphone vendors such as Samsung may bundle custom software.
Functionality focus
The basic feature set of media players are a seek bar, a timer with the current and total playback time, playback controls (play, pause, previous, next, stop), playlists, a "repeat" mode, and a "shuffle" (or "random") mode for curiosity and to facilitate searching long timelines of files.
Different media players have different goals and feature sets. Video players are a group of media players that have their features geared more towards playing digital video. For example, Windows DVD Player exclusively plays DVD-Video discs and nothing else. Media Player Classic can play individual audio and video files but many of its features such as color correction, picture sharpening, zooming, set of hotkeys, DVB support and subtitle support are only useful for video material such as films and cartoons. Audio players, on the other hand, specialize in digital audio. For example, AIMP exclusively plays audio formats. MediaMonkey can play both audio and video formats, but many of its features including media library, lyric discovery, music visualization, online radio, audiobook indexing, and tag editing are geared toward consumption of audio material; watching video files on it can be a trying feat. General-purpose media players also do exist. For example, Windows Media Player has exclusive features for both audio and video material, although it cannot match the feature set of Media Player Classic and MediaMonkey combined. | Media player software | Wikipedia | 505 | 160223 | https://en.wikipedia.org/wiki/Media%20player%20software | Technology | Computer software | null |
By default, videos are played with fully visible field of view while filling at least either width or height of the viewport to appear as large as possible. Options to change the video's scaling and aspect ratio may include filling the viewport through either stretching or cropping, and "100% view" where each pixel of the video covers exactly one pixel on the screen.
Zooming into the field of view during playback may be implemented through a slider on any screen or with pinch zoom on touch screens, and moving the field of view may be implemented through scrolling by dragging inside the view port or by moving a rectangle inside a miniature view of the entire field of view that denotes the magnified area.
Media player software may have the ability to adjust appearance and acoustics during playback using effects such as mirroring, rotating, cropping, cloning, adjusting colours, deinterlacing, and equalizing and visualizing audio. Easter eggs may be featured, such as a puzzle game on VLC Media Player.
Still snapshots may be extracted directly from a video frame or captured through a screenshot, the former of which is preferred since it preserves videos' original dimensions (height and width). Video players may show a tooltip bubble previewing footage at the position hovered over with the mouse cursor.
A preview tooltip for the seek bar has been implemented on few smartphones through a stylus or a self-capacitive touch screen able to detect a floating finger. Such include the Samsung Galaxy S4, S5 (finger), Note 2, Note 4 (stylus), and Note 3 (both).
Streaming media players may indicate buffered segments of the media in the seek bar. | Media player software | Wikipedia | 348 | 160223 | https://en.wikipedia.org/wiki/Media%20player%20software | Technology | Computer software | null |
3D video players
3D video players are used to play 2D video in 3D format. A high-quality three-dimensional video presentation requires that each frame of a motion picture be embedded with information on the depth of objects present in the scene. This process involves shooting the video with special equipment from two distinct perspectives or modeling and rendering each frame as a collection of objects composed of 3D vertices and textures, much like in any modern video game, to achieve special effects. Tedious and costly, this method is only used in a small fraction of movies produced worldwide, while most movies remain in the form of traditional 2D images. It is, however, possible to give an otherwise two-dimensional picture the appearance of depth. Using a technique known as anaglyph processing a "flat" picture can be transformed so as to give an illusion of depth when viewed through anaglyph glasses (usually red-cyan). An image viewed through anaglyph glasses appears to have both protruding and deeply embedded objects in it, at the expense of somewhat distorted colors. The method itself is old enough, dating back to the mid-19th century, but it is only with recent advances in computer technology that it has become possible to apply this kind of transformation to a series of frames in a motion picture reasonably fast or even in real-time, i.e. as the video is being played back. Several implementations exist in the form of 3D video players that render conventional 2D video in anaglyph 3D, as well as in the form of 3D video converters that transform video into stereoscopic anaglyph and transcode it for playback with regular software or hardware video players.
Examples
Well known examples of media player software include Windows Media Player, VLC media player, iTunes, Winamp, Media Player Classic, MediaMonkey, foobar2000, AIMP, MusicBee and JRiver Media Center. Most of these also include music library managers.
Although media players are often multi-media, they can be primarily designed for a specific media. For example, Media Player Classic and VLC media player are video-focused while Winamp and iTunes are music-focused, despite all of them supporting both types of media.
Home theater PC | Media player software | Wikipedia | 444 | 160223 | https://en.wikipedia.org/wiki/Media%20player%20software | Technology | Computer software | null |
A home theater PC or media center computer is a convergence device that combines some or all the capabilities of a personal computer with a software application that supports video, photo, audio playback, and sometimes video recording functionality. Although computers with some of these capabilities were available from the late 1980s, the "Home Theater PC" term first appeared in mainstream press in 1996. Since 2007, other types of consumer electronics, including gaming systems and dedicated media devices have crossed over to manage video and music content. The term "media center" also refers to specialized computer programs designed to run on standard personal computers. | Media player software | Wikipedia | 117 | 160223 | https://en.wikipedia.org/wiki/Media%20player%20software | Technology | Computer software | null |
Carbon fibers or carbon fibres (alternatively CF, graphite fiber or graphite fibre) are fibers about in diameter and composed mostly of carbon atoms. Carbon fibers have several advantages: high stiffness, high tensile strength, high strength to weight ratio, high chemical resistance, high-temperature tolerance, and low thermal expansion. These properties have made carbon fiber very popular in aerospace, civil engineering, military, motorsports, and other competition sports. However, they are relatively expensive compared to similar fibers, such as glass fiber, basalt fibers, or plastic fibers.
To produce a carbon fiber, the carbon atoms are bonded together in crystals that are more or less aligned parallel to the fiber's long axis as the crystal alignment gives the fiber a high strength-to-volume ratio (in other words, it is strong for its size). Several thousand carbon fibers are bundled together to form a tow, which may be used by itself or woven into a fabric.
Carbon fibers are usually combined with other materials to form a composite. For example, when permeated with a plastic resin and baked, it forms carbon-fiber-reinforced polymer (often referred to as carbon fiber), which has a very high strength-to-weight ratio and is extremely rigid although somewhat brittle. Carbon fibers are also composited with other materials, such as graphite, to form reinforced carbon-carbon composites, which have a very high heat tolerance.
Carbon fiber-reinforced materials are used to make aircraft and spacecraft parts, racing car bodies, golf club shafts, bicycle frames, fishing rods, automobile springs, sailboat masts, and many other components where light weight and high strength are needed.
History
In 1860, Joseph Swan produced carbon fibers for the first time, for use in light bulbs. In 1879, Thomas Edison baked cotton threads or bamboo slivers at high temperatures carbonizing them into an all-carbon fiber filament used in one of the first incandescent light bulbs to be heated by electricity. In 1880, Lewis Latimer developed a reliable carbon wire filament for the incandescent light bulb, heated by electricity. | Carbon fibers | Wikipedia | 427 | 160277 | https://en.wikipedia.org/wiki/Carbon%20fibers | Technology | Fabrics and fibers | null |
In 1958, Roger Bacon created high-performance carbon fibers at the Union Carbide Parma Technical Center located outside of Cleveland, Ohio. Those fibers were manufactured by heating strands of rayon until they carbonized. This process proved to be inefficient, as the resulting fibers contained only about 20% carbon. In the early 1960s, a process was developed by Dr. Akio Shindo at Agency of Industrial Science and Technology of Japan, using polyacrylonitrile (PAN) as a raw material. This had produced a carbon fiber that contained about 55% carbon. In 1960 Richard Millington of H.I. Thompson Fiberglas Co. developed a process (US Patent No. 3,294,489) for producing a high carbon content (99%) fiber using rayon as a precursor. These carbon fibers had sufficient strength (modulus of elasticity and tensile strength) to be used as a reinforcement for composites having high strength to weight properties and for high temperature resistant applications.
The high potential strength of carbon fiber was realized in 1963 in a process developed by W. Watt, L. N. Phillips, and W. Johnson at the Royal Aircraft Establishment at Farnborough, Hampshire. The process was patented by the UK Ministry of Defence, then licensed by the British National Research Development Corporation to three companies: Rolls-Royce, who were already making carbon fiber; Morganite; and Courtaulds. Within a few years, after successful use in 1968 of a Hyfil carbon-fiber fan assembly in the Rolls-Royce Conway jet engines of the Vickers VC10, Rolls-Royce took advantage of the new material's properties to break into the American market with its RB-211 aero-engine with carbon-fiber compressor blades. Unfortunately, the blades proved vulnerable to damage from bird impact. This problem and others caused Rolls-Royce such setbacks that the company was nationalized in 1971. The carbon-fiber production plant was sold off to form Bristol Composite Materials Engineering Ltd (often referred to as Bristol Composites). | Carbon fibers | Wikipedia | 416 | 160277 | https://en.wikipedia.org/wiki/Carbon%20fibers | Technology | Fabrics and fibers | null |
In the late 1960s, the Japanese took the lead in manufacturing PAN-based carbon fibers. A 1970 joint technology agreement allowed Union Carbide to manufacture Japan's Toray Industries product. Morganite decided that carbon-fiber production was peripheral to its core business, leaving Courtaulds as the only big UK manufacturer. Courtaulds's water-based inorganic process made the product susceptible to impurities that did not affect the organic process used by other carbon-fiber manufacturers, leading Courtaulds ceasing carbon-fiber production in 1991.
During the 1960s, experimental work to find alternative raw materials led to the introduction of carbon fibers made from a petroleum pitch derived from oil processing. These fibers contained about 85% carbon and had excellent flexural strength. Also, during this period, the Japanese Government heavily supported carbon fiber development at home and several Japanese companies such as Toray, Nippon Carbon, Toho Rayon and Mitsubishi started their own development and production. Since the late 1970s, further types of carbon fiber yarn entered the global market, offering higher tensile strength and higher elastic modulus. For example, T400 from Toray with a tensile strength of 4,000 MPa and M40, a modulus of 400 GPa. Intermediate carbon fibers, such as IM 600 from Toho Rayon with up to 6,000 MPa were developed. Carbon fibers from Toray, Celanese and Akzo found their way to aerospace application from secondary to primary parts first in military and later in civil aircraft as in McDonnell Douglas, Boeing, Airbus, and United Aircraft Corporation planes. In 1988, Dr. Jacob Lahijani invented balanced ultra-high Young's modulus (greater than 100 Mpsi) and high tensile strength pitch carbon fiber (greater than 500 kpsi) used extensively in automotive and aerospace applications. In March 2006, the patent was assigned to the University of Tennessee Research Foundation.
Structure and properties | Carbon fibers | Wikipedia | 392 | 160277 | https://en.wikipedia.org/wiki/Carbon%20fibers | Technology | Fabrics and fibers | null |
Carbon fiber is frequently supplied in the form of a continuous tow wound onto a reel. The tow is a bundle of thousands of continuous individual carbon filaments held together and protected by an organic coating, or size, such as polyethylene oxide (PEO) or polyvinyl alcohol (PVA). The tow can be conveniently unwound from the reel for use. Each carbon filament in the tow is a continuous cylinder with a diameter of 5–10 micrometers and consists almost exclusively of carbon. The earliest generation (e.g. T300, HTA and AS4) had diameters of 16–22 micrometers. Later fibers (e.g. IM6 or IM600) have diameters that are approximately 5 micrometers.
The atomic structure of carbon fiber is similar to that of graphite, consisting of sheets of carbon atoms arranged in a regular hexagonal pattern (graphene sheets), the difference being in the way these sheets interlock. Graphite is a crystalline material in which the sheets are stacked parallel to one another in regular fashion. The intermolecular forces between the sheets are relatively weak Van der Waals forces, giving graphite its soft and brittle characteristics.
Depending upon the precursor to make the fiber, carbon fiber may be turbostratic or graphitic, or have a hybrid structure with both graphitic and turbostratic parts present. In turbostratic carbon fiber the sheets of carbon atoms are haphazardly folded, or crumpled, together. Carbon fibers derived from polyacrylonitrile (PAN) are turbostratic, whereas carbon fibers derived from mesophase pitch are graphitic after heat treatment at temperatures exceeding 2200 °C. Turbostratic carbon fibers tend to have high ultimate tensile strength, whereas heat-treated mesophase-pitch-derived carbon fibers have high Young's modulus (i.e., high stiffness or resistance to extension under load) and high thermal conductivity.
Applications
Carbon fiber can have higher cost than other materials which has been one of the limiting factors of adoption. In a comparison between steel and carbon fiber materials for automotive materials, carbon fiber may be 10-12x more expensive. However, this cost premium has come down over the past decade from estimates of 35x more expensive than steel in the early 2000s. | Carbon fibers | Wikipedia | 482 | 160277 | https://en.wikipedia.org/wiki/Carbon%20fibers | Technology | Fabrics and fibers | null |
Composite materials
Carbon fiber is most notably used to reinforce composite materials, particularly the class of materials known as carbon fiber or graphite reinforced polymers. Non-polymer materials can also be used as the matrix for carbon fibers. Due to the formation of metal carbides and corrosion considerations, carbon has seen limited success in metal matrix composite applications. Reinforced carbon-carbon (RCC) consists of carbon fiber-reinforced graphite, and is used structurally in high-temperature applications. The fiber also finds use in filtration of high-temperature gases, as an electrode with high surface area and impeccable corrosion resistance, and as an anti-static component. Molding a thin layer of carbon fibers significantly improves fire resistance of polymers or thermoset composites because a dense, compact layer of carbon fibers efficiently reflects heat.
The increasing use of carbon fiber composites is displacing aluminum from aerospace applications in favor of other metals because of galvanic corrosion issues. Note, however, that carbon fiber does not eliminate the risk of galvanic corrosion. In contact with metal, it forms "a perfect galvanic corrosion cell ..., and the metal will be subjected to galvanic corrosion attack" unless a sealant is applied between the metal and the carbon fiber.
Carbon fiber can be used as an additive to asphalt to make electrically conductive asphalt concrete. Using this composite material in the transportation infrastructure, especially for airport pavement, decreases some winter maintenance problems that lead to flight cancellation or delay due to the presence of ice and snow. Passing current through the composite material 3D network of carbon fibers dissipates thermal energy that increases the surface temperature of the asphalt, which is able to melt ice and snow above it.
Textiles | Carbon fibers | Wikipedia | 350 | 160277 | https://en.wikipedia.org/wiki/Carbon%20fibers | Technology | Fabrics and fibers | null |
Precursors for carbon fibers are polyacrylonitrile (PAN), rayon and pitch. Carbon fiber filament yarns are used in several processing techniques: the direct uses are for prepregging, filament winding, pultrusion, weaving, braiding, etc. Carbon fiber yarn is rated by the linear density (weight per unit length; i.e., 1 g/1000 m = 1 tex) or by number of filaments per yarn count, in thousands. For example, 200 tex for 3,000 filaments of carbon fiber is three times as strong as 1,000 carbon filament yarn, but is also three times as heavy. This thread can then be used to weave a carbon fiber filament fabric or cloth. The appearance of this fabric generally depends on the linear density of the yarn and the weave chosen. Some commonly used types of weave are twill, satin and plain. Carbon filament yarns can also be knitted or braided.
Microelectrodes
Carbon fibers are used for fabrication of carbon-fiber microelectrodes. In this application typically a single carbon fiber with diameter of 5–7 μm is sealed in a glass capillary. At the tip the capillary is either sealed with epoxy and polished to make a carbon-fiber disk microelectrode, or the fiber is cut to a length of 75–150 μm to make a carbon-fiber cylinder electrode. Carbon-fiber microelectrodes are used either in amperometry or fast-scan cyclic voltammetry for detection of biochemical signaling.
Flexible heating
Despite being known for their electrical conductivity, carbon fibers can carry only very low currents on their own. When woven into larger fabrics, they can be used to reliably provide (infrared) heating in applications requiring flexible electrical heating elements and can easily sustain temperatures past 100 °C. Many examples of this type of application can be seen in DIY heated articles of clothing and blankets. Due to its chemical inertness, it can be used relatively safely amongst most fabrics and materials; however, shorts caused by the material folding back on itself will lead to increased heat production and can lead to a fire.
Synthesis | Carbon fibers | Wikipedia | 459 | 160277 | https://en.wikipedia.org/wiki/Carbon%20fibers | Technology | Fabrics and fibers | null |
Each carbon filament is produced from a polymer such as polyacrylonitrile (PAN), rayon, or petroleum pitch. All these polymers are known as a precursor. For synthetic polymers such as PAN or rayon, the precursor is first spun into filament yarns, using chemical and mechanical processes to initially align the polymer molecules in a way to enhance the final physical properties of the completed carbon fiber. Precursor compositions and mechanical processes used during spinning filament yarns may vary among manufacturers. After drawing or spinning, the polymer filament yarns are then heated to drive off non-carbon atoms (carbonization), producing the final carbon fiber. The carbon fibers filament yarns may be further treated to improve handling qualities, then wound on to bobbins.
A common method of manufacture involves heating the spun PAN filaments to approximately 300 °C in air, which breaks many of the hydrogen bonds and oxidizes the material. During this process, fibers tend to shrink. The resulting chemical composition and mechanical properties of the fiber are dependent on the time and temperature of the process, as well as on the tension applied to the fiber during oxidation. The oxidized PAN is then placed into a furnace having an inert atmosphere of a gas such as argon, and heated to approximately 2000 °C, which induces graphitization of the material, changing the molecular bond structure. When heated in the correct conditions, these chains bond side-to-side (ladder polymers), forming narrow graphene sheets which eventually merge to form a single, columnar filament. The result is usually 93–95% carbon. Lower-quality fiber can be manufactured using pitch or rayon as the precursor instead of PAN. The carbon can become further enhanced, as high modulus, or high strength carbon, by heat treatment processes. Carbon heated in the range of 1500–2000 °C (carbonization) exhibits the highest tensile strength (5,650MPa, or 820,000psi), while carbon fiber heated from 2500 to 3000 °C (graphitizing) exhibits a higher modulus of elasticity (531GPa, or 77,000,000psi). | Carbon fibers | Wikipedia | 446 | 160277 | https://en.wikipedia.org/wiki/Carbon%20fibers | Technology | Fabrics and fibers | null |
In astronomy and celestial navigation, an ephemeris (; ; , ) is a book with tables that gives the trajectory of naturally occurring astronomical objects and artificial satellites in the sky, i.e., the position (and possibly velocity) over time. Historically, positions were given as printed tables of values, given at regular intervals of date and time. The calculation of these tables was one of the first applications of mechanical computers. Modern ephemerides are often provided in electronic form. However, printed ephemerides are still produced, as they are useful when computational devices are not available.
The astronomical position calculated from an ephemeris is often given in the spherical polar coordinate system of right ascension and declination, together with the distance from the origin if applicable. Some of the astronomical phenomena of interest to astronomers are eclipses, apparent retrograde motion/planetary stations, planetary es, sidereal time, positions for the mean and true nodes of the moon, the phases of the Moon, and the positions of minor celestial bodies such as Chiron.
Ephemerides are used in celestial navigation and astronomy. They are also used by astrologers. GPS signals include ephemeris data used to calculate the position of satellites in orbit.
History | Ephemeris | Wikipedia | 255 | 160332 | https://en.wikipedia.org/wiki/Ephemeris | Technology | Astronomical technology | null |
1st millennium BC – Ephemerides in Babylonian astronomy.
2nd century AD – the Almagest and the Handy Tables of Ptolemy
8th century AD – the of Ibrāhīm al-Fazārī
9th century AD – the of Muḥammad ibn Mūsā al-Khwārizmī
11th century AD – the of Ibn Yunus
12th century AD – the Tables of Toledo – based largely on Arabic sources of Islamic astronomy – were edited by Gerard of Cremona to form the standard European ephemeris until the Alfonsine Tables.
13th century AD – the Zīj-i Īlkhānī (Ilkhanic Tables) were compiled at the Maragheh observatory in Persia.
13th century AD – the Alfonsine Tables were compiled in Spain to correct anomalies in the Tables of Toledo, remaining the standard European ephemeris until the Prutenic Tables almost 300 years later.
13th century AD - the Dresden Codex, an extant Mayan ephemeris
1408 – Chinese ephemeris table (copy in Pepysian Library, Cambridge, UK (refer book '1434'); Chinese tables believed known to Regiomontanus).
1474 – Regiomontanus publishes his day-to-day Ephemerides in Nürnberg, Germany.
1496 – the Almanach Perpetuum of Abraão ben Samuel Zacuto (one of the first books published with a movable type and printing press in Portugal)
1504 – While shipwrecked on the island of Jamaica, Christopher Columbus successfully predicted a lunar eclipse for the natives, using the ephemeris of the German astronomer Regiomontanus.
1531 – Work of Johannes Stöffler is published posthumously at Tübingen, extending the ephemeris of Regiomontanus through 1551.
1551 – the Prutenic Tables of Erasmus Reinhold were published, based on Copernicus's theories. | Ephemeris | Wikipedia | 388 | 160332 | https://en.wikipedia.org/wiki/Ephemeris | Technology | Astronomical technology | null |
1554 – Johannes Stadius published Ephemerides novae et auctae, the first major ephemeris computed according to Copernicus' heliocentric model, using parameters derived from the Prutenic Tables. Although the Copernican model provided an elegant solution to the problem of computing apparent planetary positions (it avoided the need for the equant and better explained the apparent retrograde motion of planets), it still relied on the use of epicycles, leading to some inaccuracies – for example, periodic errors in the position of Mercury of up to ten degrees. One of the users of Stadius's tables is Tycho Brahe.
1627 – the Rudolphine Tables of Johannes Kepler based on elliptical planetary motion became the new standard.
1679 – La Connaissance des Temps ou calendrier et éphémérides du lever & coucher du Soleil, de la Lune & des autres planètes, first published yearly by Jean Picard and still extant.
1975 – Owen Gingerich, using modern planetary theory and digital computers, calculates the actual positions of the planets in the 16th century and graphs the errors in the planetary positions predicted by the ephemerides of Stöffler, Stadius and others. According to Gingerich, the error patterns "are as distinctive as fingerprints and reflect the characteristics of the underlying tables. That is, the error patterns for Stöffler are different from those of Stadius, but the error patterns of Stadius closely resemble those of Maestlin, Magini, Origanus, and others who followed the Copernican parameters." | Ephemeris | Wikipedia | 339 | 160332 | https://en.wikipedia.org/wiki/Ephemeris | Technology | Astronomical technology | null |
Modern ephemeris
For scientific uses, a modern planetary ephemeris comprises software that generates positions of planets and often of their satellites, asteroids, or comets, at virtually any time desired by the user.
After introduction of electronic computers in the 1950s it became feasible to use numerical integration to compute ephemerides. The Jet Propulsion Laboratory Development Ephemeris is a prime example. Conventional so-called analytical ephemerides that utilize series expansions for the coordinates have also been developed, but of much increased size and accuracy as compared to the past, by making use of computers to manage the tens of thousands of terms. Ephemeride Lunaire Parisienne and VSOP are examples.
Typically, such ephemerides cover several centuries, past and future; the future ones can be covered because the field of celestial mechanics has developed several accurate theories. Nevertheless, there are secular phenomena which cannot adequately be considered by ephemerides. The greatest uncertainties in the positions of planets are caused by the perturbations of numerous asteroids, most of whose masses and orbits are poorly known, rendering their effect uncertain. Reflecting the continuing influx of new data and observations, NASA's Jet Propulsion Laboratory (JPL) has revised its published ephemerides nearly every year since 1981.
Solar System ephemerides are essential for the navigation of spacecraft and for all kinds of space observations of the planets, their natural satellites, stars, and galaxies.
Scientific ephemerides for sky observers mostly contain the positions of celestial bodies in right ascension and declination, because these coordinates are the most frequently used on star maps and telescopes. The equinox of the coordinate system must be given. It is, in nearly all cases, either the actual equinox (the equinox valid for that moment, often referred to as "of date" or "current"), or that of one of the "standard" equinoxes, typically J2000.0, B1950.0, or J1900. Star maps almost always use one of the standard equinoxes.
Scientific ephemerides often contain further useful data about the moon, planet, asteroid, or comet beyond the pure coordinates in the sky, such as elongation to the Sun, brightness, distance, velocity, apparent diameter in the sky, phase angle, times of rise, transit, and set, etc.
Ephemerides of the planet Saturn also sometimes contain the apparent inclination of its ring. | Ephemeris | Wikipedia | 510 | 160332 | https://en.wikipedia.org/wiki/Ephemeris | Technology | Astronomical technology | null |
Celestial navigation serves as a backup to satellite navigation. Software is widely available to assist with this form of navigation; some of this software has a self-contained ephemeris. When software is used that does not contain an ephemeris, or if no software is used, position data for celestial objects may be obtained from the modern Nautical Almanac or Air Almanac.
An ephemeris is usually only correct for a particular location on the Earth. In many cases, the differences are too small to matter. However, for nearby asteroids or the Moon, they can be quite important.
Other modern ephemerides recently created are the EPM (Ephemerides of Planets and the Moon), from the Russian Institute for Applied Astronomy of the Russian Academy of Sciences, and the INPOP () by the French IMCCE. | Ephemeris | Wikipedia | 171 | 160332 | https://en.wikipedia.org/wiki/Ephemeris | Technology | Astronomical technology | null |
In statistics, quality assurance, and survey methodology, sampling is the selection of a subset or a statistical sample (termed sample for short) of individuals from within a statistical population to estimate characteristics of the whole population. The subset is meant to reflect the whole population and statisticians attempt to collect samples that are representative of the population. Sampling has lower costs and faster data collection compared to recording data from the entire population, and thus, it can provide insights in cases where it is infeasible to measure an entire population.
Each observation measures one or more properties (such as weight, location, colour or mass) of independent objects or individuals. In survey sampling, weights can be applied to the data to adjust for the sample design, particularly in stratified sampling. Results from probability theory and statistical theory are employed to guide the practice. In business and medical research, sampling is widely used for gathering information about a population. Acceptance sampling is used to determine if a production lot of material meets the governing specifications.
History
Random sampling by using lots is an old idea, mentioned several times in the Bible. In 1786, Pierre Simon Laplace estimated the population of France by using a sample, along with ratio estimator. He also computed probabilistic estimates of the error. These were not expressed as modern confidence intervals but as the sample size that would be needed to achieve a particular upper bound on the sampling error with probability 1000/1001. His estimates used Bayes' theorem with a uniform prior probability and assumed that his sample was random. Alexander Ivanovich Chuprov introduced sample surveys to Imperial Russia in the 1870s.
In the US, the 1936 Literary Digest prediction of a Republican win in the presidential election went badly awry, due to severe bias . More than two million people responded to the study with their names obtained through magazine subscription lists and telephone directories. It was not appreciated that these lists were heavily biased towards Republicans and the resulting sample, though very large, was deeply flawed. | Sampling (statistics) | Wikipedia | 400 | 160361 | https://en.wikipedia.org/wiki/Sampling%20%28statistics%29 | Mathematics | Statistics and probability | null |
Elections in Singapore have adopted this practice since the 2015 election, also known as the sample counts, whereas according to the Elections Department (ELD), their country's election commission, sample counts help reduce speculation and misinformation, while helping election officials to check against the election result for that electoral division. The reported sample counts yield a fairly accurate indicative result with a 95% confidence interval at a margin of error within 4-5%; ELD reminded the public that sample counts are separate from official results, and only the returning officer will declare the official results once vote counting is complete.
Population definition
Successful statistical practice is based on focused problem definition. In sampling, this includes defining the "population" from which our sample is drawn. A population can be defined as including all people or items with the characteristics one wishes to understand. Because there is very rarely enough time or money to gather information from everyone or everything in a population, the goal becomes finding a representative sample (or subset) of that population.
Sometimes what defines a population is obvious. For example, a manufacturer needs to decide whether a batch of material from production is of high enough quality to be released to the customer or should be scrapped or reworked due to poor quality. In this case, the batch is the population.
Although the population of interest often consists of physical objects, sometimes it is necessary to sample over time, space, or some combination of these dimensions. For instance, an investigation of supermarket staffing could examine checkout line length at various times, or a study on endangered penguins might aim to understand their usage of various hunting grounds over time. For the time dimension, the focus may be on periods or discrete occasions.
In other cases, the examined 'population' may be even less tangible. For example, Joseph Jagger studied the behaviour of roulette wheels at a casino in Monte Carlo, and used this to identify a biased wheel. In this case, the 'population' Jagger wanted to investigate was the overall behaviour of the wheel (i.e. the probability distribution of its results over infinitely many trials), while his 'sample' was formed from observed results from that wheel. Similar considerations arise when taking repeated measurements of properties of materials such as the electrical conductivity of copper. | Sampling (statistics) | Wikipedia | 454 | 160361 | https://en.wikipedia.org/wiki/Sampling%20%28statistics%29 | Mathematics | Statistics and probability | null |
This situation often arises when seeking knowledge about the cause system of which the observed population is an outcome. In such cases, sampling theory may treat the observed population as a sample from a larger 'superpopulation'. For example, a researcher might study the success rate of a new 'quit smoking' program on a test group of 100 patients, in order to predict the effects of the program if it were made available nationwide. Here the superpopulation is "everybody in the country, given access to this treatment" – a group that does not yet exist since the program is not yet available to all.
The population from which the sample is drawn may not be the same as the population from which information is desired. Often there is a large but not complete overlap between these two groups due to frame issues etc. (see below). Sometimes they may be entirely separate – for instance, one might study rats in order to get a better understanding of human health, or one might study records from people born in 2008 in order to make predictions about people born in 2009.
Time spent in making the sampled population and population of concern precise is often well spent because it raises many issues, ambiguities, and questions that would otherwise have been overlooked at this stage.
Sampling frame
In the most straightforward case, such as the sampling of a batch of material from production (acceptance sampling by lots), it would be most desirable to identify and measure every single item in the population and to include any one of them in our sample. However, in the more general case this is not usually possible or practical. There is no way to identify all rats in the set of all rats. Where voting is not compulsory, there is no way to identify which people will vote at a forthcoming election (in advance of the election). These imprecise populations are not amenable to sampling in any of the ways below and to which we could apply statistical theory.
As a remedy, we seek a sampling frame which has the property that we can identify every single element and include any in our sample. The most straightforward type of frame is a list of elements of the population (preferably the entire population) with appropriate contact information. For example, in an opinion poll, possible sampling frames include an electoral register and a telephone directory. | Sampling (statistics) | Wikipedia | 462 | 160361 | https://en.wikipedia.org/wiki/Sampling%20%28statistics%29 | Mathematics | Statistics and probability | null |
A probability sample is a sample in which every unit in the population has a chance (greater than zero) of being selected in the sample, and this probability can be accurately determined. The combination of these traits makes it possible to produce unbiased estimates of population totals, by weighting sampled units according to their probability of selection.
Example: We want to estimate the total income of adults living in a given street. We visit each household in that street, identify all adults living there, and randomly select one adult from each household. (For example, we can allocate each person a random number, generated from a uniform distribution between 0 and 1, and select the person with the highest number in each household). We then interview the selected person and find their income.
People living on their own are certain to be selected, so we simply add their income to our estimate of the total. But a person living in a household of two adults has only a one-in-two chance of selection. To reflect this, when we come to such a household, we would count the selected person's income twice towards the total. (The person who is selected from that household can be loosely viewed as also representing the person who isn't selected.)
In the above example, not everybody has the same probability of selection; what makes it a probability sample is the fact that each person's probability is known. When every element in the population does have the same probability of selection, this is known as an 'equal probability of selection' (EPS) design. Such designs are also referred to as 'self-weighting' because all sampled units are given the same weight.
Probability sampling includes: simple random sampling, systematic sampling, stratified sampling, probability-proportional-to-size sampling, and cluster or multistage sampling. These various ways of probability sampling have two things in common:
Every element has a known nonzero probability of being sampled and
involves random selection at some point.
Nonprobability sampling | Sampling (statistics) | Wikipedia | 407 | 160361 | https://en.wikipedia.org/wiki/Sampling%20%28statistics%29 | Mathematics | Statistics and probability | null |
Nonprobability sampling is any sampling method where some elements of the population have no chance of selection (these are sometimes referred to as 'out of coverage'/'undercovered'), or where the probability of selection cannot be accurately determined. It involves the selection of elements based on assumptions regarding the population of interest, which forms the criteria for selection. Hence, because the selection of elements is nonrandom, nonprobability sampling does not allow the estimation of sampling errors. These conditions give rise to exclusion bias, placing limits on how much information a sample can provide about the population. Information about the relationship between sample and population is limited, making it difficult to extrapolate from the sample to the population.
Example: We visit every household in a given street, and interview the first person to answer the door. In any household with more than one occupant, this is a nonprobability sample, because some people are more likely to answer the door (e.g. an unemployed person who spends most of their time at home is more likely to answer than an employed housemate who might be at work when the interviewer calls) and it's not practical to calculate these probabilities.
Nonprobability sampling methods include convenience sampling, quota sampling, and purposive sampling. In addition, nonresponse effects may turn any probability design into a nonprobability design if the characteristics of nonresponse are not well understood, since nonresponse effectively modifies each element's probability of being sampled.
Sampling methods
Within any of the types of frames identified above, a variety of sampling methods can be employed individually or in combination. Factors commonly influencing the choice between these designs include:
Nature and quality of the frame
Availability of auxiliary information about units on the frame
Accuracy requirements, and the need to measure accuracy
Whether detailed analysis of the sample is expected
Cost/operational concerns
Simple random sampling | Sampling (statistics) | Wikipedia | 391 | 160361 | https://en.wikipedia.org/wiki/Sampling%20%28statistics%29 | Mathematics | Statistics and probability | null |
In a simple random sample (SRS) of a given size, all subsets of a sampling frame have an equal probability of being selected. Each element of the frame thus has an equal probability of selection: the frame is not subdivided or partitioned. Furthermore, any given pair of elements has the same chance of selection as any other such pair (and similarly for triples, and so on). This minimizes bias and simplifies analysis of results. In particular, the variance between individual results within the sample is a good indicator of variance in the overall population, which makes it relatively easy to estimate the accuracy of results.
Simple random sampling can be vulnerable to sampling error because the randomness of the selection may result in a sample that does not reflect the makeup of the population. For instance, a simple random sample of ten people from a given country will on average produce five men and five women, but any given trial is likely to over represent one sex and underrepresent the other. Systematic and stratified techniques attempt to overcome this problem by "using information about the population" to choose a more "representative" sample.
Also, simple random sampling can be cumbersome and tedious when sampling from a large target population. In some cases, investigators are interested in research questions specific to subgroups of the population. For example, researchers might be interested in examining whether cognitive ability as a predictor of job performance is equally applicable across racial groups. Simple random sampling cannot accommodate the needs of researchers in this situation, because it does not provide subsamples of the population, and other sampling strategies, such as stratified sampling, can be used instead.
Systematic sampling
Systematic sampling (also known as interval sampling) relies on arranging the study population according to some ordering scheme and then selecting elements at regular intervals through that ordered list. Systematic sampling involves a random start and then proceeds with the selection of every kth element from then onwards. In this case, k=(population size/sample size). It is important that the starting point is not automatically the first in the list, but is instead randomly chosen from within the first to the kth element in the list. A simple example would be to select every 10th name from the telephone directory (an 'every 10th' sample, also referred to as 'sampling with a skip of 10'). | Sampling (statistics) | Wikipedia | 475 | 160361 | https://en.wikipedia.org/wiki/Sampling%20%28statistics%29 | Mathematics | Statistics and probability | null |
As long as the starting point is randomized, systematic sampling is a type of probability sampling. It is easy to implement and the stratification induced can make it efficient, if the variable by which the list is ordered is correlated with the variable of interest. 'Every 10th' sampling is especially useful for efficient sampling from databases.
For example, suppose we wish to sample people from a long street that starts in a poor area (house No. 1) and ends in an expensive district (house No. 1000). A simple random selection of addresses from this street could easily end up with too many from the high end and too few from the low end (or vice versa), leading to an unrepresentative sample. Selecting (e.g.) every 10th street number along the street ensures that the sample is spread evenly along the length of the street, representing all of these districts. (If we always start at house #1 and end at #991, the sample is slightly biased towards the low end; by randomly selecting the start between #1 and #10, this bias is eliminated.)
However, systematic sampling is especially vulnerable to periodicities in the list. If periodicity is present and the period is a multiple or factor of the interval used, the sample is especially likely to be unrepresentative of the overall population, making the scheme less accurate than simple random sampling.
For example, consider a street where the odd-numbered houses are all on the north (expensive) side of the road, and the even-numbered houses are all on the south (cheap) side. Under the sampling scheme given above, it is impossible to get a representative sample; either the houses sampled will all be from the odd-numbered, expensive side, or they will all be from the even-numbered, cheap side, unless the researcher has previous knowledge of this bias and avoids it by a using a skip which ensures jumping between the two sides (any odd-numbered skip).
Another drawback of systematic sampling is that even in scenarios where it is more accurate than SRS, its theoretical properties make it difficult to quantify that accuracy. (In the two examples of systematic sampling that are given above, much of the potential sampling error is due to variation between neighbouring houses – but because this method never selects two neighbouring houses, the sample will not give us any information on that variation.) | Sampling (statistics) | Wikipedia | 485 | 160361 | https://en.wikipedia.org/wiki/Sampling%20%28statistics%29 | Mathematics | Statistics and probability | null |
As described above, systematic sampling is an EPS method, because all elements have the same probability of selection (in the example given, one in ten). It is not 'simple random sampling' because different subsets of the same size have different selection probabilities – e.g. the set {4,14,24,...,994} has a one-in-ten probability of selection, but the set {4,13,24,34,...} has zero probability of selection.
Systematic sampling can also be adapted to a non-EPS approach; for an example, see discussion of PPS samples below.
Stratified sampling
When the population embraces a number of distinct categories, the frame can be organized by these categories into separate "strata." Each stratum is then sampled as an independent sub-population, out of which individual elements can be randomly selected. The ratio of the size of this random selection (or sample) to the size of the population is called a sampling fraction. There are several potential benefits to stratified sampling.
First, dividing the population into distinct, independent strata can enable researchers to draw inferences about specific subgroups that may be lost in a more generalized random sample.
Second, utilizing a stratified sampling method can lead to more efficient statistical estimates (provided that strata are selected based upon relevance to the criterion in question, instead of availability of the samples). Even if a stratified sampling approach does not lead to increased statistical efficiency, such a tactic will not result in less efficiency than would simple random sampling, provided that each stratum is proportional to the group's size in the population.
Third, it is sometimes the case that data are more readily available for individual, pre-existing strata within a population than for the overall population; in such cases, using a stratified sampling approach may be more convenient than aggregating data across groups (though this may potentially be at odds with the previously noted importance of utilizing criterion-relevant strata).
Finally, since each stratum is treated as an independent population, different sampling approaches can be applied to different strata, potentially enabling researchers to use the approach best suited (or most cost-effective) for each identified subgroup within the population. | Sampling (statistics) | Wikipedia | 461 | 160361 | https://en.wikipedia.org/wiki/Sampling%20%28statistics%29 | Mathematics | Statistics and probability | null |
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