even from a large symphony or-
chestra, should be picked up by micro-
phones, transmitted over telephone
wires, and reproduced at a distant
point. Most of us probably hear it
accomplished every day by means of
the radio, and radio transmission and
reproduction would, in general, be
called good. Between good reproduc-
tion and perfect, however, there is a

crossing it are probably not realized
by those not technically familiar
with the subject. Perfect reproduc-
tion, of a symphony concert for ex-
ample, would make it impossible for
one listening with his eyes blindfolded
to know that the actual orchestra was
not on a stage before him. Not only
would every tone and over-tone be
present in its correct relative volume,
but there would be a depth and color
which is not ordinarily obtained when
electrical apparatus intervenes

between theorchestraand theaudience.
Three classes of requirements must
be met if the reproduced sound is to
be indistinguishable from the original.
Two of them, that both the complete
frequency and complete volume ranges
be transmitted, have been generally
recognized for some time. The third,
that the sounds must be reproduced
with the correct auditory perspective,
has been fully appreciated only by
those most closely associated with the
science of sound reproduction.
Sounds in general are composed of a
group of tones and over-tones ranging
from the deep bass of the lowest organ
notes, or those of a bass drum, to the
shrillest tones the ear can hear. Each
note of a musical instrument has a
fundamental tone and a group of
harmonics. The fundamental tone
sets the pitch, and the harmonics give
the note its quality. It is the har-
monics that make it possible to dis-
tinguish a note on a violin from one on
a trumpet or from any other of the
same pitch. It is in the harmonics that
reside the richness of music and the
wealth of sensuous appeal. These
tones and over-tones are known and
recognized by their frequency, or
vibratory rate; and the range of fre-
quencies to which the ear responds
runs from about 16 cycles per second
to 16,000, or even 20,000 cycles for
some ears. The sensitivity of the ear
falls off rapidly at the higher fre-
quencies, however, so that the effect
of frequencies above 15,000 cycles is
negligible for the most part. The
highest note on the piano has a funda-
mental frequency of only about 4,000
cycles, and few of the musical instru-
ments exceed this pitch, but the ac-
companying harmonics or over-tones,
which are of still higher frequencies,
are very necessary to the proper
quality and richness of the notes.

Of no less importance, if the full
aesthetic effect of music is to be ob-
tained, is the range in volume. The
ear has a recognizable range of volume
as it has of frequency. This extends
from sounds so low that the ear cannot
hear them, to sounds so great that the
sensation is one of pain rather than of
hearing. For convenience in scientific
study, the power of sounds is graded
in units known as decibels (abbrevi-
ated db). The threshold of hearing is
taken as a reference base, and the
ordinary audible range runs from the
volume of sound one would hear in a
quiet garden, or that of an average
whisper at a distance of four feet,
which are at a level of 20 db, to that
of a pneumatic riveter, at a level of
100 db—a total range of about 80 db.
The range of a large symphony or-
chestra is about 70 db, so if the music
of such an orchestra is to be faithfully
transmitted electrically, a volume
range of the order of 70 db must be
transmitted: a range of power of ten
million to one.

The third requirement becomes of
particular importance when the sound
to be transmitted and reproduced is
that from a large and relatively widely
spaced group of instruments, such as
a complete symphony orchestra.
When one sits in an auditorium and
listens to a symphony concert he ex-
periences something that is over and
above the effect produced by the
actual frequency and volume range
given out by the orchestra. This ad-
ditional appeal is difficult to describe,
and almost impossible to measure. It
is partly due to a spreading of the
sound in all directions so that it fills
the entire volume of the auditorium
and thus reaches one’s ears by various
paths. It is partly due to other factors;
but whatever its cause it results in a
richness and texture of tone that no

ordinary electrical reproduction can
provide. For lack of a better term, the
effect may be called auditory per-
spective. Without it the music would
be one dimensional and not expanded
into its true spatial relationship. The
difference may be compared to that
between the appearance of a photo-
graph of a scene and the same scene
when viewed through a stereoscope.

How to obtain this auditory per-
spective in music transmitted and re-
produced electrically was discovered
by the scientists of Bell Telephone
Laboratories as a result of their funda-
mental investigations in acoustics and
telephonic transmission. During the
course of those investigations they
had developed telephonic systems of
high quality, but for their further re-
searches they needed opportunity to
utilize music in its most perfect forms.
Now it happened that Dr. Leopold
Stokowski, Director of the Philadel-
phia Orchestra, was interested in the
possibilities of electrical systems for
the production of exceptional or-
chestral effects. Through his volun-
tary cooperation, therefore, the Lab-
oratories’ scientists were able to make
quantitative physical studies of music
as rendered by his orchestra, and so to
perfect their designs; and with the
completion of the new equipment
some of the possibilities which Dr.
Stokowski had hoped for became
practicable. An extended series of
tests was then carried on in Phila-
delphia in which the Laboratories’
scientists were generously assisted by
Dr. Stokowski. As a result of these
studies, it was found that by employ-
ing two microphones, one properly
located on each side of the stage, and
by transmitting over two separate
circuits to two of the newly developed
loud speakers, similarly placed, the
effect of the actual presence of the

Even with the discovery of a com-
paratively simple means of obtaining
true auditory perspective, the prob-
lem was not completely solved. Never
before had either the complete fre-
quency, or the complete volume range,
of a symphony orchestra been com-
mercially transmitted and_ repro-
duced. No complete chain of appa-
ratus, from microphone to loud
speaker, was available that would
faithfully transmit the entire range of
frequency and volume. Microphones
perhaps offered the fewest difficulties.
Bell Laboratories had already de-
signed sensitive microphones that
would transmit practically the entire
range required, and only minor modi-
fications were needed to make them
entirely suitable.

This was not true of the amplifiers.
There had to be developed amplifiers
which would faithfully transmit all
frequencies from 35 to 16,000 cycles
at levels from the barely audible
pianissimo effects to the resounding
orchestral crashes of ten million times
greater power; and all the pieces of
apparatus had to be so designed that
even during intervals of complete
silence not the slightest noise would
be introduced to suggest the presence
of electrical apparatus. No underlying
hum or noise, such as is commonly
present in radio or other systems of
reproduction, could be tolerated with
the new apparatus. In the intervals of
silence there must be real silence: a
dead auditory void in which the fall
of the lightest pin could be heard.
This has actually been accomplished
to a degree heretofore unknown.
Probably the most quiet electrical re-
production up to the present is that
obtained with high-grade sound pic-
ture apparatus; but such apparatus at
its most quiet moments gives off 300

Control room in the basement of the Academy of Music in Philadelphia. W. A.
Munson of the Laboratories is standing at the voltage amplifiers

Of even greater difficulty possibly
was the design of suitable loud
speakers. It is not practicable to ob-
tain the entire frequency range with a
single unit, and so two types of loud
speakers are used. One, somewhat
resembling the horns used for sound
pictures, is employed for the frequen-
cies from 35 to 300 cycles; and another
type, for the range from 300 to 16,000
cycles. These loud speakers are dif-
ferent from anything previously pro-
duced commercially. Never before
have these elements fulfilled such
dificult requirements of frequency
range and volume. The best sound
picture systems record and reproduce
approximately half the range of fre-
quencies handled by the new loud
speakers, and the best radio systems
even less. In volume range the com-
parison is equally remarkable. Al-
though sound picture systems under
the most favorable conditions may
provide a volume range of 40 or 45

db, radio systems rarely exceed 30,
while the range provided by the new
apparatus is well above 80. Whereas
the power range of radio is of the
order of 1000 to 1, the new equipment
is capable of yielding a range of
100,000,000 to I.

The new loud speakers and their
associated equipment of amplifiers
and microphones are, therefore, fully
capable of handling the entire volume
range of a symphony orchestra. When
one speaks of range of loudness which
can be handled by an electrical
system for reproduction, one is con-
cerned with the differences between
the loudest and faintest passages of
the music which it can reproduce.
There is in addition the problem of
handling the peaks of maximum loud-
ness. These peaks in the case of music
from a symphony orchestra are be-
yond the possibilities of the ordinary
loud speaker to reproduce without
distortions which seriously affect the
musical sonority. The low frequency
sounds make the largest contribution

to the peaks of sound power which
must be handled to meet these con-
ditions. The diaphragm of the low
frequency element in the new loud
speaker has been made nearly seven
times larger than that of the elements
used ordinarily for sound picture re-
production. By these diaphragms a
large column of air is set into motion.

The ordinary loud speaker also be-
comes directional in its characteristics
at the higher frequencies. Low fre-
quency sounds spread in all directions
from the mouth of the horn, but the
higher frequencies tend to concen-
trate into a beam projected directly
ahead of the horn; and the width of
the beam becomes narrower and
narrower as the frequency increases.

Because of this fact, the audience, in a
large hall equipped with the ordinary
loud speakers, never hear quite the
proper blending of frequencies. Those
directly in front of the horn receive
too great a proportion of the higher
frequencies, while those on the sides
receive too much of the low frequen-
cies. To avoid this effect, the horn of
each high-frequency element is di-
vided into 16 diverging rectangular
sections which spread the sound over
an arc of 60 degrees vertically and
one of 60 degrees horizontally. Two of
these units placed side by side thus
spread the sound over a horizontal
angle of 120 degrees--a far wider
coverage than has been obtained be-
fore and one which distributes the
sound throughout the
auditorium with a
faithful blending of the
frequencies.

R. . Tillman of A. T. & T. (left), and A. R. Soffel of the 200 watts. This addi-
Laboratories at the voltage amplifiers in Constitution Hall, tional gain allows ef-

which have been impossible before.
Besides the effects of range and
quality of tone, the total aesthetic
appeal of an orchestra is due in no
small degree to the range in volume.
The number of musicians one can
place on a stage is limited. To put
ten times as many as contained in a
modern symphony orchestra is im-
possible in any existing hall. The con-
trol of volume given by the new ap-
paratus enables the director to secure
at will the equivalent of an orchestra
of nearly a thousand musicians.

The advantage of this control of
volume does not end here, however.
Its presence makes it possible to re-
produce operatic music, where a
soloist is accompanied by an orchestra,
without allowing the voice of the
singer to be drowned out by the
louder passages. For this purpose a
third channel, including its separate
microphone, transmission line, and
loud speaker, has been provided in
the new system primarily for the
singer. The volume of output of this
channel is controllable independently
of the other two. In this way the
loudness of the voice may always be
kept just above that of the orchestra
and the desired musical effect be ob-
tained. There thus reside in the newap-
paratus possibilities heretofore unat-
tainable; and telephonic research has
laid a foundation for what may be one
of the greatest advances in musical
aesthetics of the present scientific era.

The first public demonstration of
the new apparatus was given in
Washington on the evening of April
27 under the auspices of the National
Academy of Sciences. At that time
Dr. Stokowski, Director of the Phila-
delphia Orchestra, manipulated the
controls from a box in the rear of
Constitution Hall, while the Phila-
delphia Orchestra, led by Associate

Conductor Alexander Smallens, played
in the Academy of Music at Phila-
delphia. Between Philadelphia and
Washington, the music was trans-
mitted over telephone cable circuits.
The program consisted of the Toccata
and Fugue in D Minor, of Bach;
Beethoven’s Symphony No. 5 in C
Minor; L’aprés-midi d’un Faune, of
Debussy; and the Finale of Gétter-
dammerung. A visual accompaniment
was provided for the music by
Electrical Research Products, Incor-
porated. Its stage direction—through
the courtesy of the Yale School of
Drama—was by S. R. McCandless,
and the designs were by Eugene
Savage and George Davidson.
During the intermission Dr. W. W.
Campbell, President of the National
Academy of Sciences, introduced Dr.
Harvey Fletcher, Director of Acous-
tical Research at Bell Telephone
Laboratories. With the assistance of
the orchestra in Philadelphia, Dr.
Fletcher then performed several ex-
periments to demonstrate the im-
portant characteristics of the new
apparatus. On the stage of the Acad-
emy of Music in Philadelphia, where
the pickup microphones were in-
stalled, a workman busily construct-
ing a box with hammer and saw was
receiving suggestions and comments
from a fellow workman in the right
wing. All the speech and accompany-
ing sounds were transmitted over
cable circuits to the loud speakers on
the stage of Constitution Hall in
Washington. So realistic was the
effect that to the audience the act
seemed to be taking place on the
stage before them. Not only were the
sounds of sawing, hammering and
talking faithfully reproduced, but the
correct auditory perspective enabled
the listeners to place each sound in its
proper position, and to follow the

audience heard a soprano sing ““Com-
ing Through the Rye” as she walked
back and forth through an imaginary
rye field on the stage in Philadelphia.
Here again her voice was reproduced
in Washington with such exact audi-
tory perspective that the singer ap-
peared to be strolling on the stage of
Constitution Hall.

An experiment which demonstrated
both the complete fidelity of repro-
duction and the effect of auditory
perspective was performed by two
trumpet players. One, in Philadelphia
at the left of the stage of the Academy
of Music, and the other in Washing-
ton at the right of the stage of Con-
stitution Hall but invisible to the
audience, alternately played a few
phrases of the same selection. To
those in the audience there seemed to
be a trumpet player at each side of
the stage before them. It was not
until after the stage was lighted that
they realized that only one of the
trumpet players was there in person.
The music of the other was trans-
mitted from Philadelphia with such
perfect fidelity and reproduced in
such true perspective that it was im-
possible to tell that one of the players
was absent.

The auditory perspective effect is
not restricted to placing sounds in
their correct positions across the
stage, but is three dimensional. This
was shown by having several sources
of sound moved around the stage in
Philadelphia, not only back and forth
but high up in the center of the
stage as well. The movement of each
sound was faithfully reproduced by
the loud speakers in Washington even
when the sounds were carried high
above the level of the stage floor.

To show the volume range possible
with the new equipment, the orches.
tra played a selection at a constant
level of loudness while the output of
the loud speakers was varied from a
level so low that the instruments could
scarcely be heard, up to a loudness
almost great enough to be painful,
Throughout the whole range, the re-
production was faithful in all re
spects except the level of loudness;
there was no distortion or noise to
mar the perfection of the reproduc-
tion, and the wide range in volume ©
was vividly impressed on the audience.

The effect of limiting the range in
pitch, or frequency, was illustrated
by employing electric filters to cut
out one octave at a time—first from
the upper end of the range and then
from the lower. The new apparatus
reproduces faithfully about 9 octaves
or from 35 to 16,000 cycles, com-
pared to about six for ordinary radio
reproduction. By this demonstration
the audience had the opportunity of
judging the importance of the com-
plete range to the full aesthetic appeal
of music, and of comparing it with the
more limited ranges ordinarily heard.

The technical features of these new
developments, which were carried on
as part of the research program of the
American Telephone and Telegraph
Company, were disclosed for the first
time by Dr. F. B. Jewett, Vice Presi-
dent of the American Telephone and
Telegraph Company in charge of
development and research, at a meet-
ing of the National Academy of
Sciences on Tuesday afternoon of
April 25. In discussing the future of
the new system, Dr. Jewett said:

“As to the future of the accom-
plishment shown here today, it is
difficult to make any definite predic-
tion. What we have done is to pro-
duce pickup microphones, amplifiers,

electrical filters, transmission lines
and loud-speaking reproducers so
perfect that the entire frequency and
volume range of the most exacting
orchestral and vocal music can be re-
produced at a distance without im-
pairment of quality. We have also
worked out the arrangements by
which substantially perfect auditory
perspective is possible. This latter is
an essential part of the problem if
realistic illusion as to the physical
arrangement of the component parts
of an orchestra is desired.

“We can place at the disposal of the
musical director instrumentalities
which will enable him to produce at a
distant point, or at many distant
points simultaneously, a completely
faithful replica of the tonal effects
produced locally in the auditorium
on the stage of which the orchestra is
performing. Likewise, portions of
this same equipment place at his dis-
posal the means of very greatly ex-
tending the range of orchestral repro-
duction and of making possible ar-
tistic effects hitherto unattainable.

“With these instrumentalities avail-
able, the questions of the manner and
extent of their use are primarily ques-
tions for the musician and those in-
terested in music rather than for the
physicist and the engineer. Our job
has been to produce a set of tools.
The musicians and musical directors,
and back of them the musical com-
posers, must determine just how these
tools can best be used and what they
can best produce. By its very nature
the ensemble of what we have created
is primarily of value for musical pro-
duction or reproduction in_ halls,
theatres or auditoriums. In a word, its
field of applicability is where a large
number of people might congregate
for the common enjoyment of music
of distinction. In its present form it is

‘These new tools offer not only an
enlarged field of possibility to the
musician and the composer for the
production of auditory effects, but
likewise a great broadening of the

D. T. Bell of the Laboratories at the power
amplifier panels in Washington

audience which derives pleasure from
such effects. Many people, especially
in our smaller cities, are now deprived
of the ability to hear good orchestral
music by the factors of cost and
distance, and the element of time in
going to the cities where orchestral
music is normally produced. What we
as physicists and engineers have done
is to provide a mechanism for obviat-
ing these factors. Whether the results
justify our hopes is for others to say.”

onstration before the National Academy of Sciences
on _ Twenty-seventh, 1933, at Washington,

years of generally reduced com-
mercial activity, has steadily increased
its volume of business. Graphs show-
ing miles flown and passengers car-
ried both present a striking contrast
to a plot of the general index of
industrial production. Along with
this rapid growth in volume of trans-
port business has gone an increase in
cruising speed, and even greater
speeds are an immediate prospect.
Radio aids, by making flying safer
and times of departure and arrival
more certain, have contributed in no
small degree to the general success of
the industry. Such aids are, in general,
of two types: the beacon signals and
weather reports of the Department of

Commerce, and the radio telephone
systems operated by the transport
companies themselves. The growth of
both of these types of service has
paralleled that of the air transport
industry itself.

however, the transport communica-
tion picture has changed considerably
with the large increase in the number
of planes equipped for radio telephony.
The frequency band available for air-
plane telephony is strictly limited,
and the increased demand for chan-
nels has made it necessary to squeeze
the assignments closer together. With
the old equipment this has resulted in
interference between channels oper-
ated by different transport organiza-
tions. An imperative need has thus
arisen for receivers of greater selec-
tivity.

Another feature which it seemed
highly desirable to incorporate in the
new apparatus is the ability to change
the frequency easily. Radio trans-
mission conditions at night are dif-
ferent from those during the day so
that two frequencies are universally
employed: one in the vicinity of five
megacycles best adapted to day con-

ditions and the other, about three
megacycles, best adapted to night.
The equipment which has been avail-
able to the air transport industry
previously has not been arranged so
that the frequency could be changed
during flight. It sometimes happened,
therefore, that during transition per-
iods between day and night fre-
quencies, some of the planes with
which a certain ground station had to

communicate would be set for one
frequency and others for the other.
Under such conditions regular com-
munication with airplanes became
disrupted and position reports were
missed.

Frequent conferences have been
held between representatives of the
larger transport companies, and the
Western Electric Company and engi-
neers of the Laboratories, and a close
agreement has been reached regarding
the requirements for radio telephone
equipment for airplanes. As a result a
new line of radio telephone equipment
has been made available which, al-
though embodying many features of
the earlier apparatus, has a number of

novel features and is greatly improved
in many respects. Notable among
these are provisions for changing the
frequency while in flight and for
greater selectivity in the receiver, the
two problems of most immediate
concern. |

The change from day-time to night-
time frequency or vice versa is accom-
plished by the operation of switches in
the receiver and transmitter. These
are connected mechanically and oper-
ated by the pilot through a single
control. In planes where the trans-
mitter and receiver are close to the
pilot’s position, the switches are con-
trolled by a pull wire, but for greater
distances, a flexible shaft drive is em-
ployed which shifts the switch in both
receiver and transmitter at the same
time. The connection of this shaft to
the base structure of the apparatus is
shown in Figure 3. This arrangement
permits the shift between day and
night conditions to be made at pre-
established times regardless of where
the plane happens to be.

Except for this frequency shifting
arrangement and certain simplifica-
tions, the control is similar to that for
the earlier equipment. The equipment
is made ready for operation by two

control switches. When one of these jg
thrown the two receivers are placed
in operating condition, and when the
other is thrown, the transmitter js
made ready for operation. In either
case, an interphone circuit is available
between pilot and co-pilot. For this
service the audio-frequency amplifier
of the receiver is used to amplify the
signals from the microphone. The
complete equipment, including two
receivers, a transmitter, and all ac-
cessories, is shown in Figure 4, where
is shown also the flexible conduit used
for the wiring between the various
pieces of apparatus.

dynamotors have now been assembled
with their accessory equipment on a
detachable base, which makes instal-

Fig. 3—A single flexible shaft drives, through a worm and segment, the frequency
changing switch in both transmitter and receiver

Fig. 4—The 208A radio telephone equipment. The main pieces of apparatus, from
left to right, are: dynamotor power unit, controls, weather-and-beacon receiver, trans-
mitter, receiver for two-way system

In the other and newer type, power
is obtained from a double voltage
engine-driven unit, shown in Figure 5,
which may be employed as either a
generator or dynamotor at the pilot’s
volition. Under normal operation the
unit is operated through a centrifugal
clutch as an engine-driven generator
supplying both low voltage, for the
filament and control circuits, and
high voltage for the plate circuits of
the transmitter. There is also sufficient
capacity in the low voltage windings
to keep the airplane storage battery
in a charged condition. When the
engine is running below some definite
low speed, the clutch disengages and
the pilot may pull a control handle
which changes the electrical connec-
tions to enable it to be operated by
the battery as a dynamotor. Under
these conditions, and with a fully
charged 65 ampere-hour battery, the
radio equipment may be operated for
two hours with the receiver con-
tinuously in service and the trans-
mitter operating on a cycle of one

minute on and five minutes off. With
a smaller—35 ampere-hour—bat-
tery, which is adequate for the new
equipment, a similar cycle may be
maintained for one hour.

The present airplane equipment in-
cludes two fixed antennas. One of
these is used for the low frequency
receiver over which weather reports
and beacon signals are received, and
the other for the high frequency
channel—both for transmitting and
receiving. A push button on the new
anti-noise microphone, developed by
the Research Department and em-
ployed as part of the new telephone
equipment, is employed to operate
relays in the receiver and transmitter
which connect the antenna to either
receiver or transmitter as required
and make the other circuit changes
necessary for the transfer between
talk and receive conditions.

Considerable attention has been
given to making all the various pieces
of apparatus readily removable for
inspection, repairs, or replacement.
All the major units—the transmitter

and both of the receivers—are pro-
vided with separable bases which are
permanently secured to the plane and
to which the wiring is run. In these
bases the wiring terminates in re-
ceptacles, and the radio units them-
selves have plugs which fit in the re-
ceptacles when the units are pushed
into place. The base of the power unit
has plugs at the ends of short flexible
conduit connections which allow it
also to be readily removed if desired.

Although the new equipment pro-
vides for multiple-frequency opera-
tion, and offers many other new
features, it is possible for a transport
operator to install it with the new
power supply to weigh, complete,
about fifteen per cent less than the
equipment formerly available. Con-

siderable savings will also follow from
the convenient mountings and wirin
arrangements, and from the smaller
number of units to be installed.
Maintenance savings can also be ex.
pected, due to the accessibility of ad-
justment points when the apparatus
units have been removed, as they
readily can be, to the service shop.
The cordial cooperation of the air
transport companies, not only in the
design of the new apparatus but in
the more than seventy million miles of
flight of the earlier models, has con-
tributed notably to the excellence of
this radio equipment. It may reason-
ably be expected that it will be an im-
portant factor in the reliability and
safety on which the transport industry
basesitsexpectation for futureprogress,

ment during the last few years
has served to emphasize the need for
facilities permitting rapid change of
the operating frequency. The large
increase in night flying has made
general the use by each transport
company of at least two frequencies:
one best suited to day conditions and
one, to night. It is apparent that the
transition from day to night frequency
will result in confusion unless all
stations on the system, including
planes in flight, can change frequency
simultaneously. In a new airplane

transmitter which has been developed,
therefore, provision is made for chang-
ing the frequency by operating a
simple manual control. This allows
the pilot to change from day to night
frequency or vice versa at a preestab-
lished time. No technical skill is
required for this procedure as it in-
volves no tuning operations whatever.

Besides this feature, the new trans-
mitter—known as the 13-A—in-
corporates a third frequency channel
which may be selected by the same
control that changes from day to night
frequencies. This arrangement con-
tributes greatly to safety because all

Department of Commerce stations
keep constant watch on 3105 ke,
which frequency is not assigned to
any transport company. By being
able to transmit on 3105 kc, therefore,
an airplane pilot can communicate at
any time with a Department of Com-
merce station to ask for weather re-
ports or other information or to
request assistance in emergencies.
These government stations reply on
the weather broadcast frequency so
the pilot can have two way communi-
cation with them at any time without
requiring an additional channel in his
high-frequency receiver. These out-
standing improvements, as well as
others in the control and maintenance
features, arise from certain novel
refinements both in the electrical
circuits and in the mechanical
design.

The principal circuit features of the
new transmitter are shown in the
simplified schematic of Figure 1. For
clearness only one frequency channel
is shown and the dc circuits are
omitted. The radio frequency cir-
cuits consist of a crystal controlled
oscillator, and two stages of amplifica-

Fig. 2—A combination of coil and mica
condenser forming the antenna coupling
circuit is built as a single plug-in unit

tion employing screen grid tubes. The
use of these four-element tubes—
especially designed for this service by
H. E. Mendenhall—eliminates the
delicate and troublesome neutralizing
adjustment, which is very advanta-
geous in portable apparatus.

Fig. 1—Simplified schematic of transmitter showing only one frequency channel and

In developing methods and providing
apparatus for the reproduction of sym-
phonic music with its original quality and
in true auditory perspective, the com-
bined efforts of many engineers of Bell
Laboratories have been required. The
necessary telephone repeaters and termi-
nal equipment were developments of
H. S. Black and R. W. Chesnut. Loud
speakers and microphones were developed
under the direction of Dr. E. C. Wente
and A. L. Thuras. The provision of suit-
able amplifiers with the necessary equa-
lizers and volume controls was undertaken
by E. O. Scriven, R. A. Miller, and A. F.
Price. The acoustic and electrical re-

searches have in part been carried on by
tests in Philadelphia with the codperation
of Dr. Stokowski. The conduct of the
tests has largely been the work of Dr.
J. C. Steinberg and W. B. Snow of Bell
Laboratories. The engineers of the Ameri-
can Telephone and Telegraph Company
solved the several problems of trans-
mitting over cable circuits ranges of
volume and pitch never before attempted.
For their solution H. A. Affel and R. G.
McCurdy were largely responsible.
* * *

Dr. Fewett holds the diaphragm of the low-frequency loud speaker, while Dr. Fletcher
shows where the driving coil fits on the diaphragm

April twelfth. Seated in the auditorium
and facing the stage hidden by its curtain,
music critics and press representatives
heard selections played by the Philadel-
phia Symphony Orchestra under the di-
rection of Dr. Leopold Stokowski, and
reproduced by loud speakers from the
empty stage. Although the orchestra
members were playing in the foyer, the
music sounded as if they were in their
regular places on the stage. The capa-
bilities of the new apparatus which made
possible this remarkable illusion were
demonstrated by Dr. Harvey Fletcher
by a set of experiments similar to those
given in Washington and described in the
accompanying article.

The usual artist’s palette consists of a
dozen or more pigments, with the result
that the number of ways of making de-
sired colors by mixture is very large and
quite unamenable tosystemization. Learn-
ing how to use these numerous pigments
is a matter of long experience. The simpli-
fication indicated by the three-color prin-
ciple has been retarded in realization

largely owing to the mistaken, but widely
held, belief that the primary pigment
colors are red, yellow and blue. Actually
the pigment primaries, which act by sub-
traction or absorption of light from white,
should be complementary in hue to the
red, green and blue which are the pri-
maries for mixing light by addition. Those
colors are: a minus red, (spectrum minus
red) or turquoise; a minus green, or
crimson; and a minus blue, or yellow,
each having wide overlapping spectral re-
flection bands. Pigments of these colors,
of proper spectral characteristics, are
capable of mixing in pairs to make red,
green and blue, and all three together to
make black. When mixed with white all
variations of saturation and hue are ob-
tained. |

The practical problem consists in pro-
curing pigments possessing the indicated
spectral reflectivities, and having satis-
factory chemical properties, such as free-
dom from reaction with the oil or other
medium, and satisfactory permanence.
Due to the very great advances which
have been made in the dye industry to
meet recent demands for permanent
colors for automobiles and outdoor signs,
it is now possible to select pigments nearly
enough meeting the scientific require-
ments to test the practicability of the
principle. This has been done with success,
and Dr. Ives exhibited pictures so painted,
in connection with the presentation of his
paper.

The great advantage of a three-color
palette is its simplicity and freedom from
ambiguity. Only three colors (and white)
are needed in place of the large number
ordinarily used by artists. All colors can
be obtained from these, in just one way,
which is indicated by a simple and easily
grasped theory. The perfection and adop-
tion of a three-color palette will open the
way to the teaching of color to artists on
a scientific instead of an empirical basis.

On MarcH 20 Dr. G. A. Campbell of the
American Telephone and Telegraph Com-
pany spoke before the Colloquium on

Systems of Units and Their Unification. Dr.
Campbell has recently concerned himself
with the familiar annoyance of the multiple
systems of electrical units, and pointed
out in his address the road to a solution.

On apriv 3 Dr. I. I. Rabi of Columbia
University gave an address on Dvirect
Measurement of Nuclear Angular Mo-
mentum. During a stay at Hamburg some
years ago, Dr. Rabi collaborated with Pro-
fessor Stern (who addressed the Collo-
quium two years ago) in developing the
methods of producing and detecting nar-
row straight beams of neutral atoms and
molecules and of sending them across
non-uniform magnetic fields in order to
determine the magnetic moments of the
particles. In previous experiments the
particles of these beams have the Max-
wellian distribution, in velocity corre-
sponding to the temperature of the furnace
whence they come. As a result, the several
pencils into which such a beam is split by
a non-uniform magnetic field are broader
and hazier than is desirable. Dr. Rabi per-
fected a technique for sending a beam
through two fields in succession. After
going through the first field, one of the
broadened pencils is intercepted by a
screen with a narrow slit which allows
only particles of a narrow range of
speeds to pass so that it functions as a
velocity-filter. The beam thus filtered out
is sent through a second and much
weaker non-uniform field, and is sub-
divided into several pencils much finer
and sharper than had earlier been ob-
tained. The manner of subdivision gives
information about the nuclear spin of the
kind of atom or molecule in question. As
estimates of the spin can also be made
from features of band-spectra and from
hyperfine structure of line spectra, inter-
esting comparisons are possible. Dr. Rabi
described details of the method and
further inferences from the data.

Meet1nG aT the Deal Laboratories on
April 6, the Radio Colloquium heard Mr.
R. C. Mathes speak on Volume Control
Circuits.

SEVERAL members of the Laboratories
took an active part in the scientific con.
ference and dinner held at the Masgsa-
chusetts Institute of Technology on
March 29 to celebrate the eightieth birth-
day of Elihu Thomson, distinguished engi-
neer and inventor. The committee in
charge of arrangements included Dr,
Jewett and Mr. Charlesworth. Among the
speakers at the conference in the after.
noon was Dr. K. K. Darrow, who dis-
cussed the trends of research, and at the
dinner in the evening Mr. Charlesworth,
as President of the A.I.E.E., spoke for the
engineering societies.

To the Review of Scientific Instruments
K. K. Darrow has contributed an account
of Neutrons in February and one of New
Achievements in Transmutation in March.
A. W. Kishpaugh and R. E. Coram have
written on Low Power Radio Transmitters

for Broadcasting in the February issue of
the Proceedings of the Institute of Radio
Engineers. A paper by F. A. Polkinghorn
on Short-Wave Transoceanic Telephone
Receiving Equipment, which first appeared
in the Proceedings of the Radio Club of
America, has been reprinted in Radio
Engineering for February. The same
month, in the Journal of the Society of
Motion Picture Engineers, E. F. Kings-
bury reviews a book on Photocells and
Their Applications by V. K. Zworykin
and E. D. Wilson. In the March issue of
the Proceedings of the Institute of Radio
Engineers appear Some Results of a Study
of Ultra-Short-Wave Transmission Phe-
nomena by C. R. Englund, A. B. Craw-
ford, and W. W. Mumford, and an article
on Ultra-Short-W ave Propagation by J. C.
Schelleng, C. R. Burrows and E. B.
Ferrell. The Journal of the Optical Society
for March contains H. E. Ives and T. C.
Fry’s article on Standing Light Waves;
Repetition of an Experiment by Wiener
Using a Photoelectric Probe Surface.
March also saw the publication of Pu/p—
The New Cable Insulation by L. S. Ford in

Wire, an abstract of H. A. Frederick and
H. C. Harrison’s paper on Vertically Cut
Sound Records in Electrical Engineering,
and W. H. Brattain and J. A. Becker’s
account of Thermionic and Adsorption
Characteristics of Thorium on Tungsten
in the Physical Review.

In addition to the articles mentioned
in last month’s issue of the Recorp,
several others appeared in January publi-
cations. The Journal of the Acoustical
Society of America contained an article
by R. R. Riesz on The Relationship Be-
tween Loudness and the Minimum Per-
ceptible Increment of Intensity, and a re-
view by R. L. Wegel of A. L. Kimball’s
book, Vibration Prevention in Engineering.
John Mills’ paper on Technical Exposition
for the General Reader has been printed in
the Journal of Engineering Education,
and his description of The Bell System
Exhibit at the “Century of Progress” Ex-
position appears in the Bell Telephone
Quarterly. The recently issued Annals of
Otology, Rhinology, and Laryngology for
September, 1932, contains Harvey

Fletcher’s answer to the question, Can We
Scientifically Advise Patients as to the

Effectiveness of Hearing Aids? and R. L.
Wegel’s account of Physical Data and
Physiology of Excitation of the Auditory
Nerve.

In The Engineer's Manual of English,
by W. O. Sypherd and Sharon Brown,
recently published by Scott, Foresman
and Company, The Magic of Communica-
tion by John Mills is quoted to furnish
“an admirable example of effective non-
technical English’; and his address, Some
Universal Principles of Communication
is reprinted as a model for technical
presentation. In the same text book
the Western Electric instruction bulle-
tin on the No. 13-A Oscillator is cited
as an example of good instruction
bulletins.

THE ANNUAL spring exhibition of the
Telephone Camera Club of Manhattan
will be on display in the club rooms at 178
Fulton Street from May 15 to 26, in-
clusive. The photographs in this exhibi-
tion are all the work of members of the
club, and several of the exhibitors are
members of the Laboratories.

G. K. Situ discussed problems arising
in connection with wiper cords on step-by-
step switches with New York Telephone
Company engineers at Albany.

R. H. Miter and F. B. Biake visited
Philadelphia to discuss a new panel
tandem project with engineers of the
Bell Telephone Company of Pennsylvania
and the Western Electric Company.

G. A. Benson visited Wilkes-Barre to
aid in the introduction of a new type of
toll ticket.

C. H. BrpweE tu inspected one of the
initial installations of No. 16 toll test
board at Bristol, Connecticut.

ON THE ELEVENTH of last month
Adolph Bregartner completed thirty-five
years of continuous service with the
Western Electric Company and the Lab-

oratories. Mr. Bregartner joined the
Western Electric Company in New York
in 1898, and after a few months in the
machine shop, entered the group handling
the assembly and adjustment of relays.

Later he worked on special apparatus,
such as the early equipment for train dis.
patching, and then became associated
with the semi-mechanical central office
development. For nearly four years he
took part in the early installations of this
equipment, in the Mulberry, Market,
and Branch Brook offices of Newark, and
since their completion he has been a
member of the staff of the dial circuit
laboratory.

W. L. Berrs visited Washington during
March to discuss several new develop-
ments with members of the Bureau of
Engineering of the Navy Department.

F.R. McMurry, incompany withE. F.
Watson of the A. T. & T. Company,
visited the Teletype Corporation in con-
nection with the manufacture of tele-
typewriter apparatus.

J. R. Barpstey and C. C. Hipxins
visited Huntington, L. I. to inspect a new
finish on underground loading coil cases.

W. E. Kau visited Washington in con-
nection with filters for high quality
program circuits.

R. W. DE Monte and D. E. Trucksgss
visited Kearny for a discussion of battery
charging rectifier equipment.

J. M. Witson, W. A. Evans, R. Burns
and K. G. CouTLee attended meetings of
Committee D-g, on Electrical Insulating
Materials, of the American Society for
Testing Materials held at the Hotel
Gramatan, Bronxville.

Roy E. Coram completed twenty
years of service in the Bell System on
the seventeenth of last month.

T. E. SHea and H. C. Curt visited
Newport News and Washington in con-
nection with Naval equipment problems.

R. M. Pease set up and operated a
public address system used during the
dinner given to Dr. Elihu Thomson at
Massachusetts Institute of Technology.
The speakers used lapel microphones con-
nected to this system.

R. A. Mitier and A. F. Price visited
Washington in connection with the trans-
mission and reproduction of the program
of the Philadelphia Symphony Orchestra.
D. T. Bell has spent the last month in
Washington and Philadelphia on the same
project.

Tuomas W. Criarke, head of the Mer-
chandise Department of the Laboratories,
died on April 8. Mr. Clarke joined the
Western Electric Company in 1906, and
after broad experience in the commercial
activities of the Engineering Department
and the Laboratories, took charge of the
branches of our General Service Depart-
ment devoted to storing, shipping, receiv-

Osvatp E. Rasmussen completed
twenty years of service in the Bell
System on the twenty-first of last month.

Dr. J. S. Waterman, A. F. WEBER
and J. S. Epwarps attended the Fourth
Annual Greater New York Safety Con-
ference held at the Hotel Pennsylvania on
March 1 and 2. W. W. Schormann acted
as a member of the Committee on Ar-
rangements for this conference.

F. D. Leamer talked on February 10
before the Graduate Physics Seminar of
New York University at Washington
Square College on Diffraction of X-Rays
in Liquids.

D. G. Btatrner visited Newport
News, Va. in connection with loud
speaker equipment for U. S. S. Ranger.

R. R. Witirams, R. M. Burns, A. R.
Kemp, J. H. Incmanson, A. E. Scuuu
and B. L. CiarKkeE attended the meetings
of the American Chemical Society in
Washington, D. C., the week of March 27.
While there Mr. Burns and C. L. Hippen-
steel presented a paper at the Bureau of
Standards Soil Corrosion Conference.

A. L. Samuet talked before a meeting
of the Radio Club of Brooklyn on the de-
sign of transmitting tubes for use at
ultra-high frequencies, and on the prob-
lems encountered at these frequencies.
He stressed the fact that this large and
unexplored field was open to amateurs
and urged them to work in it as they did
earlier in developing short wave radio on
lower frequencies. Several tubes were
exhibited for use in the wave-length
range from 12 centimeters to 3 meters.

Bancroft Gherardi and Herbert E. Ives
Are Elected Members of the
National Academy of Sciences

Bancroft Gherardi, Vice President and Chief Engineer
of the American Telephone and Telegraph Company, and
Dr. Herbert E. Ives, Electro-Optical Research Director of
these Laboratories, were elected to membership in the
National Academy of Sciences at its annual meeting in
Washington on the evening of April 25. At the same meet-
ing Dr. fewett presented a paper on the new method of
music reproduction in auditory perspective, demonstrated
to the Academy two days later and described elsewhere in
this issue of the REcorv.

The National Academy of Sciences was incorporated in
1863, by act of Congress approved by President Abraham
Lincoln. At that time its membership was limited to fifty.
Seven years later this restriction was removed, but the
membership has been kept small, and election to it remains
a signal honor. No more than fifteen members are ad-
mitted annually, and the Academy numbers less than
three hundred. Dr. Fewett and Dr. C. F. Davisson are
other members of the Bell System who have been so honored.

Fig. 3—The 13A radio telephone trans-
mitter with cover removed showing one of
the plug-in transformer units being inserted

the oscillator and amplifier, and be-
tween the two amplifier stages. These
are radio frequency transformers
which in conjunction with the tube
and wiring capacities (shown on the
diagram as dotted condensers) form
band pass filters. The two trans-
formers for each of the three frequency
channels are built as a single plug-in

ation night frequencies and another,
the day frequencies. Transformers
suited to other bands than those
used for aircraft communication are
also available. R. C. Newhouse was
largely responsible for their develop-
ment.

The crystals are also arranged in
plug-in units, and may be seen behind
the transformers in the photograph.
The crystal is connected in the grid
circuit of the oscillator tube, and
operates at one-half of the desired
output frequency. The primary of
the transformer coupling the oscil-
lator to the first amplifier presents a
high inductive reactance to the plate
of the oscillator at the crystal fre-
quency, which is necessary to produce
oscillations. The second harmonic of
the crystal frequency is passed by this
transformer and drives the first am-
plifier at output frequency. Similarly
the other transformer passes the out-
put frequency and drives the second
amplifier.

Coupling between the second ampli-
fier and the antenna is secured by a
simple tuned circuit which must be
adjusted in the field to tune various
antennas of widely different char-
acteristics. There are also three of
these circuits which, consisting of a
coil and a fixed mica condenser, are
built in the form of a plug-in unit as

shown in Figure 2 and in the front of
the transmitter in Figure 3. A con-
tinuous winding of bare tinned copper
is wound on an isolantite form, and
clips on slide rods can be set on any
turn. When clamped in place they
make good contact directly with the
winding itself. Fine tuning is done by
a small inductance wound on the in-
side of the coil form at the low po-
tential end of the coil. This contact
is adjusted with a screw-driver which
may be inserted through a small door
in the front of the transmitter.

Three-point switches are employed
to select the desired crystal, pair of
interstage transformers, and antenna
coupling units, and all the switches
are mechanically connected together
and operated by a single control. An
interlocking switch is also connected
to the same control which prevents
application of high voltage to the
transmitter except when the switches
are centered on one or another of the
three channels. This switch also
lights a lamp in the control unit near
the pilot when the switches are off
position.

A novel system of modulation is
used in the new transmitter which
permits deep modulation of the fifty
watt carrier with only about one watt
of audio-frequency power. This fea-
ture is directly responsible for the very
satisfactory overall efficiency. The
audio amplifier—like the oscillator a
205D tube—is employed to modulate
the screen bias of both of the radio
frequency amplifier stages. The over-
all characteristic of these two modu-
lating amplifiers in cascade is nearly
linear up to substantially complete
modulation. With maximum modu-
lation, the largest harmonic in the
rectified output is less than 10% of the
fundamental. The overall audio-fre-
quency characteristic of the trans-

Fig. 6— The new ballast lamp, employing a
two-contact Ediswan base, is of unusually
small size

mitter is shown in Figure 4. The low
frequencies are purposely attenuated
by a series condenser in the input
circuit to reduce the amplitude of
airplane noise picked up by the micro-
phone.

A schematic circuit of the complete
transmitter is shown in Figure 5. A
thermocouple in the radio-frequency
ground circuit is employed to operate
an antenna ammeter near the pilot,
who can therefore note whether the
current is normal and whether it
modulates when he speaks into the
microphone—a_ positive assurance
that his transmitting equipment is
working properly. All grid and plate
circuits are equipped with jacks (not
shown on the diagram) accessible
through a door on the front of the
transmitter, which facilitate the loca-
tion of trouble and routine checking
by the maintenance men. An antenna
relay, employed to switch the antenna

between the transmitter and_ re-
ceiver, is made part of the transmitter
to reduce the number of component
parts of the system. The development
of miniature ballast lamps by the
group under J. R. Wilson has made
it possible to use a lamp in each fila-
ment branch. The small size of these

Fig. 7—Separable transmitter mounting
which allows the transmitter to be readily
removed or replaced as desired

Operation of the equipment re-
quires the transmitter to stop im-
mediately when the microphone but-
ton is released and to produce no
noise in an adjacent receiver of very
high gain. This was greatly facili-
tated by the development, in coopera-
tion with V. L. Ronci, of a high
vacuum relay which opens the 1050
volt plate supply. This relay is very
small and light and is entirely safe to
use where gasoline fumes may be
present.

In the mechanical design of the
transmitter, for which P. S. March
was responsible, several novel features
are incorporated. A separate mount-
ing is designed for permanent in-
stallation in the airplane. An upright
section at the rear of the mounting,

shown in Figure 7, carries a multiple
contact jack terminating all connec.
tions to the transmitter except the
antenna, and a coupling to connect to
the switches that select the frequency
channel. All wiring is run to this
mounting in either flexible or rigid
conduit. A lever projecting from the
front operates a cam which, when the
transmitter is placed on the mounting,
slides it back into contact with the
receptacle and the frequency changing
switch.

The entire structure of the trans-
mitter itself is fabricated from sheet
duralumin, and not only supports
the apparatus but provides the
necessary shielding. It is divided in-
to two irregularly shaped compart-
ments by interior partitions. Part
of the outer casing is perforated and
the space between the inner par-
tition and the perforated section of
the outer case forms a ventilated com-
partment into which is placed only
the heat dissipating apparatus, such
as the vacuum tubes, ballast lamps,
resistances, and crystals. All other
apparatus and wiring is completely
enclosed for protection against dust
and moisture. A new type of vitreous
enamel resistor with rear connections
was developed to keep the wiring out
of the ventilated compartment.

The 13A radio transmitter is an
excellent example of the great im-
provement that can be made in
apparatus by a careful study of the
most desirable form of the various
component parts and their relative
locations based on an intimate knowl-
edge of service conditions. Even
without a radical change in funda-
mental performance, the new trans-
mitter is so superior to the old in
many respects that it is expected to
receive an enthusiastic reception from
air transport operators.

ratus indicated, among other
things, that there was need for greater
selectivity in the receiver, and that
the changes between night and day
frequencies should be easy to make
during a flight. Because of the
limited width of the frequency band
available for aviation telephone com-
munication and of the large number
of operating companies desiring fre-
quency allocations, it has been neces-
sary to assign channels with a fre-
quency separation of only about one-
tenth per cent. Since this is only one
or two tenths of the separation of
broadcast channels, the difficulty in
selecting the channel desired without
interference from the adjacent ones is
evident. The requirement of being
able to change frequencies while in
flight arises because the ground sta-

tion must be able to be in continual
communication with a group of planes,
some of which may be just beginning
a flight and others about to terminate
one. At transition periods between
day and night conditions, a plane
about to end a flight may be set for
one frequency and a plane beginning
a flight, at another. Without the
ability to change frequency at a
definite time while in the air, ready
communication with the ground neces-
sarily becomes handicapped.

In the development of the Western
Electric 12A receiver, which is a part
of the new aviation radio equipment,
these two requirements have been
fully and satisfactorily met. Certain
other improvements which a study of
past experience indicated to be de-
sirable have also been made. In spite
of these many improvements, the new
receiver will weigh no more than the

A schematic of the receiver is shown
in Figure I. To obtain the required
degree of selectivity and sensitivity,
a superheterodyne circuit 1s employed.
From left to right in the diagram,
there is one stage of tuned radio-
frequency amplification, a first de-
tector or modulator, three stages of
intermediate frequency amplification
at 385 kc, a detector and automatic
volume control tube, and one stage
of audio frequency amplification. A
separate tuned circuit for each fre-
quency is employed for the radio
frequency stage and for the oscillator.
Operation of a shifting mechanism
selects the proper circuits for the
frequency desired, the circuits having
been properly tuned while the plane
was on the ground.

In any radio receiver the usable
sensitivity is limited by the tube and
circuit noises. When this limit has
been reached, sensitivity to still
weaker signals is obtainable only by
increasing the voltage input. For a
given field strength,
the voltage induced in
a given antenna is
fixed, but the voltage
across a tuned circuit
in series with the an-
tenna can be much
greater. Antennas or-
dinarily used for air-
planes have a com-
paratively low resist-
ance, and so lend
themselves to the series
tuning which is em-
ployed in this receiver
with excellent effect.

the frequency of the beating oscillator,
thus insuring the correct frequency for
satisfactory operation under all condi-
tions and without attention on the part
of the operator. Two oscillators are
provided, and the one required is select-
ed by the operation of the same control
that selects the proper tuned circuits.

Under some operating conditions
the high degree of frequency stability
and freedom from attention on the
part of the operator provided by the
quartz plates may not be required.
The 12A receiver has therefore been
designed so that in such cases a tuning
unit, either the 8A or 8B, may be used
in place of the quartz plates. These
are plug-in tuned circuits whose con-
stants are much more stable than are
those of ordinary tuned circuits. The
coil is wound on an isolantite form,
and the adjustable condenser has a
thermostatic metal plate to reduce
variations due to temperature changes.
The units are mounted in moisture
proof aluminum cans to avoid changes
in the oscillator frequency caused by
changes in humidity.

Fig. 2— Tuned transformers in cylindrical cans are placed
adjacent to the tubes with which they are associated

These precautions, together with
the careful design of the oscillator
circuit, minimize frequency variations
caused by changes in the supply
voltage, and provide a high degree of
frequency stability. Small variations
do occur, however, and a vernier con-
denser is required to compensate for
the frequency drift. To avoid the use
of a mechanical drive for the vernier
when the receiver is controlled from a
remote point, the vernier condenser is
located at the operating point and
connected to the set through a shielded
radio-frequency transmission line of
low impedance.

To obtain the high degree of selec-
tivity required, three intermediate-
frequency amplifier tubes with four
double tuned transformers are em-
ployed. Each such double tuned trans-
former comprises a filter section and,
mounted in an aluminum shield, is
placed next to the amplifier tube, as
may be seen in Figure 2. The fourth
tuned transformer is connected to the
detector which operates as a two-
element rectifier and not only supplies
signal output, but furnishes voltage

Fig. 3—The 12A receiver showing door on front and the
shock-proof base on which it mounts

All but the audio frequency tube
are the recently developed Western
Electric 283A tubes, which have
variable mu and high gain. The de.
tector and oscillator are operated as
two and three element tubes re.
spectively by connecting certain elec-
trodes together. By this arrangement
the number of different types of tubes
is reduced. For the audio frequency
stage a 285A tube is employed. This is
a pentode capable of delivering a
sufficiently high output for the satis-
factory operation of several pairs of
headphones. When the receiver is
used as part of an airplane system,
this tube also acts as the amplifier for
the side tone circuit when the trans-
mitter is operated. A relay in the
receiver makes the necessary change
in connections when the button on
the microphone is pushed.

Automatic gain control, widely
used with present-day broadcast re-
ceivers, is even more important with
airplane receivers, since in addition to
the usual fading due to variation in
the transmission path,
there is a large change
in signal strength due
to the travel of the
plane. With the sys-
tem employed in the
12A receiver there is a
barely noticeable
change in audio output
with a variation in
signal input of 10,000
to 1. Even with this
wide range in auto-
matic control, how-
ever, a certain amount
of manual control is
desirable. An input in
excess of half a volt is
too great to be properly

handled by the automatic control, and
since voltages of this magnitude may
be applied when the plane is flying
close to the transmitter, a manual
control is provided near the operator’s
position, where it may be adjusted as
required. Under normal conditions no
adjustment is necessary during flight.
A variable level control, also, is pro-
vided to allow the operator to adjust
the headset volume to a comfortable
value.

The power required for the heaters
of the vacuum tubes is 3.2 amperes at
12 volts. A ballast lamp in the heater
supply circuit provides adequate regu-
lation for applied voltages from 11.5
to 14.5 volts. When the quartz plates
are employed for frequency control,
an additional intermittent drain of
2.4 amperes is required at the same
voltage. For the plates and screens
some 40 milliamperes is required at
200 volts, which is furnished by a
small dynamotor operated from the
battery.

To secure rigidity and still allow
easy access to the various parts of the
circuit, the apparatus is mounted on a
dished aluminum chassis, as may be
seen in Figure 2. The radio-frequency
tuning coils, the intermediate fre-
quency filters, the quartz crystal
frequency controls, the filter system
for the power supply, the various
relays, and all vacuum tubes are
mounted on the upper or flush surface
of this chassis, while the radio-fre-
quency tuning condensers, the high
frequency choke coils, and the asso-
ciated by-pass condensers are mounted
underneath on the dished side of the
chassis. An aluminum box surrounds
the chassis and apparatus to provide
mechanical protection and overall
shielding. A removable top gives ac-
cess to the tubes and frequency con-
trols, and a small hinged door on the

front gives access to the antenna
tuning condensers and the indicator
lamps of the crystal heater, as shown
in Figure 3.

The complete receiver is placed on
a base which has rubber shock proof-
ing to protect the receiver from vibra-
tion normally found in an airplane.
This mounting is permanently in-
stalled in the plane and to it is run
the power supply cable, the leads of
which terminate in a multicontact re-
ceptacle to which a plug on the re-
ceiver makes contact when the re-
ceiver is placed in the mounting. The
frequency changing mechanism is also
part of the mounting and connects
to the receiver through a coupling.
This arrangement permits the receiver
to be readily removed for repair or
replacement without requiring any

One of the outstanding charac-
teristics of this receiver is its high
value of selectivity. The response
from an interfering signal only ten
kilocycles from the desired one is
about 1/1000 of that due to the de-
sired signal. A selectivity curve is
shown in Figure 4. Although the re-
ceivers used for this service at the
ground stations have similar selec-
tivity, it far surpasses anything pre-
viously attained in airplane sets.

The sensitivity obtained in the
three stages of intermediate-frequency

amplification and one of radio-fre.
quency, is very high. An i input of one
microvolt at the antenna gives an
output of over twelve milliwatts,
which is more than _ sufficient io
headphone reception. Such high sen-
sitivity is not required for normal
operation, but it insures a sufficient
reserve of amplification to give satis.
factory reception under abnormal con-
ditions. The outstanding performance
of this receiver, together with its sim-
plicity of operation, should be of con-
siderable value in increasing the re.
liability of aviation communication.

On March 20 the telephones of Costa Rica were tied to the
Bell System by a radio link between the American Tele-
phone and Telegraph Company’s new station at Miami
and the station at Cartago, near San ‘fose, of the Interna-
tional Radio Company of Costa Rica.

On March 29 the Bell System’s radio stations in Cali-
fornia, which have been operating on the link to the
Hawaiian Islands, undertook communication with the
Philippine Islands as well. Radio facilities in the Philip-
pines are provided by the Radio Corporation of America,
whose stations tie in with the wire lines of the Philippine
Long Distance Telephone Company.

On April 7 the cities of Ferusalem, Haifa and Faffa
were placed within reach of American telephones by ar-
rangements for connecting the regular transatlantic radio-
telephone circuits with a short-wave channel between
London and Cairo, Egypt, and thus with land wire

A three-minute conversation between New York and
Palestine costs $37.50, and between New York and San
Fose, Costa Rica, $27. A similar conversation between
San Francisco and Manila costs $30.

the synthesis on a commercial
scale of several materials which were
formerly obtainable only from natural
sources. As the limit of possible
pressures has been raised, many in-
vestigators have attempted to imitate
geological processes, including those
by which coal is formed. Constituting
as it does the best material yet found
for the preparation of transmitter
carbon, the possibility of discovering
and reproducing Nature’s method of
manufacturing anthracite is alluring
to the telephone chemist.

Current theories of coal formation
have already been summarized in the
Recorp*. Past attempts to reproduce
this procedure have been at the hands
of men primarily interested in check-
ing these theories, in improving their
understanding of the formation of
bituminous coals, or in the manu-
facture of oils from coal and hydrogen.
Their reports of the formation of
anthracite-like substances by the ap-
plication of heat and pressure to vari-
ous starting materials gave encourage-
ment to these Laboratories to extend
the range of investigation, with the
production of anthracite-like coals
more definitely in mind.

The most generally accepted start-
ing material is vegetation high in
cellulose content but also containing
lignin, such as the ordinary woods.
The lignin which constitutes the

binder in such vegetation may be im-
portant in forming a binder in the
ultimate coal, but vegetation in which
the lignin content is high probably
produces the brown coals rather than
anthracite. In the formation of an-
thracite and other high rank coals
some materials other than lignin have
been important. These are supposed
to be the products resulting from the
decomposition of the cellulosic con-
stituents of the original vegetation
either by the action of bacteria or

Fig. 1—Closing the autoclave in which the
raw materials for artificial anthracite were
pretreated at low temperatures

yeasts or by the prolonged influence
of temperature and pressure. Known
to be effective in the decomposition of
vegetation, certain types of bacteria
have probably been important in the
initial chemical changes occurring in
the formation of coals. However it is
possible in the laboratory to bring
about similar changes by the effect of
temperature and pressure alone, and
at a great saving of time.

Wood begins to decompose at tem-
peratures a few degrees above 100°
Centigrade. As the temperature rises,
the decomposition increases, and at
about 300° C. a reaction takes place
accompanied by the evolution of heat
and of large quantities of gases and
liquids, chiefly water vapor, oxides of
carbon, methane and tar when no added
pressure is applied. At 400° C. under
atmospheric pressure the decomposi-
tion is complete, and the solid residue
of charcoal is nearly pure carbon.
Applying pressure to this charcoal
fails to produce anything resembling
anthracite, at least at the tempera-
tures used in this investigation, prob-
ably because the binding materials
have been destroyed.

When the same progressive heating
is carried out in a sealed tube, the re-
action products build up a consider-
able pressure, the carbonization is less
nearly complete, and the proportion
of solid and liquid to gaseous products
is higher. The residual peat-like solid,
of rather low carbon content, is
probably very similar to that formed
at an early stage in the natural con-
version of vegetation to coal. It was
therefore decided to attempt the syn-
thesis of anthracite by pretreating the
raw material so as to convert it into
something resembling peat, and then
subject this peat to higher tem-
peratures and pressures.

birch wood, cotton cloth, sugar, and
jute fiber of high lignin content. For
pretreatment the material was placed
in a special electrically heated steel
autoclave (Figure 1), along with vary-
ing quantities of water. The water
adds somewhat to the pressure de-
veloped, prevents local heating, and
at elevated temperature and pressure
provides a_ slightly acid medium
which has been suggested as having
a catalytic effect on the carboniza-
tion. Pretreatments of different
samples have covered the range of
temperatures from 180° C. to 300°C,
and of pressures from 800 to 10,000
pounds per square inch.

At temperatures up to 230° C. the
products of this pretreatment were not
at all like coal but quite like peat:
brown in color, still showing much of
the original structure, readily pow-
dered, and with a hydrogen content
of about six per cent. Neither these
nor the products of pretreatment at
higher temperature could be converted
to anthracite-like coal at the lower
temperatures of final treatment. The
gases produced during pretreatment
consisted chiefly of carbon dioxide,
mixed with amounts of carbon mon-
oxide and methane which increased
with the treating temperature. Only
small amounts of tar were produced,
far less than by heating at atmospheric
pressure.

For final treatment, charges of 100
to 200 grams of the pretreated ma-
terials were placed between two plun-
gers in a heavy steel cylinder, heated
electrically, and subjected to pressure
in a hydraulic press (Figure 2). To
extend to the utmost the range of
temperatures and pressures which
could be used, the cylinder was lined
with a special chrome-nickel steel, de-
veloped by the Society of Automotive
Engineers, which will retain good

Fig. 2—In the final step in producing experimental samples of artificial anthracite, the
materials were subjected to high temperatures and high pressures in a steel cylinder

hardness and elasticity under 100,000
pounds per square inch up to 500° C.
The clearance between the plungers
and the cylinder walls was made
ample to permit the escape of gases,
since it is believed that rock fissures
and lines of fracture permit such
escape in the natural formation of
anthracite. Final treatments in this
bomb covered a range of pressures up
to 56,700 pounds per square inch.

At the outset of the experiments
high pressure was applied to the
charges while they were being heated.
It was soon found that a strong re-
action evolving heat set in at about
320° C., accompanied by the volumi-
nous escape of gases which would
often blow most of the charge out past
the piston. In some cases a shell of coal
would be formed against the walls of
the bomb which prevented the escape
of gas, with the result that the center
of the charge was dull, porous and
crumbly. Unsatisfactory also was the
technique of applying high pressure

It was finally found best to hold
the charge under low pressure at the
final operating temperature of 400° C.
to 450° C. for about an hour, and then
to apply high pressure gradually and
to maintain this pressure during the
heating and while the charge was later
allowed to cool to room temperature.
In this manner uniform and firmly
bound products were obtained.

The majority of the products, and
the most successful, were obtained
from wood. Many of the samples
showed the lustre of natural anthra-
cite, had comparable hardness and
apparent density, and fractured to
leave conchoidal surfaces. They
burned quite slowly in a slow stream
of pure oxygen, with little or no visible
flame, and negligible evolution of tar
and coal gas. The residual ash, weigh-
ing usually less than one per cent of the
original sample, consisted mostly of
iron oxide, probably because of the

use of iron vessels in pretreatment and
final treatment. The content of car-
bon in the best samples obtained
varied from 71 to 89 per cent, and of
hydrogen from 3.9 to 3.2 per cent,
well within the reported range of
lower grade natural anthracites but
not sufficiently high in carbon nor
low in hydrogen to correspond with
anthracites of the best grade.

The hydrogen content was lower,
the higher the temperature of final
treatment; and when this content was
plotted against temperature, the
downward slope of the curve became
greater above 400 degrees Centigrade.
Greater lengths of treatment at any
fixed temperature also decreased the
hydrogen content, but the rate of de-
crease diminished as the period of
treatment was prolonged. The varia-
tion in hydrogen content was such as
to offer little encouragement to in-
creasing the pressure, and indicated
that the low hydrogen content of
certain natural anthracites must be
due to factors other than pressure
alone.

Samples of the more promising
products were ground, screened,
roasted by the methods used in pre-
paring transmitter carbon.* The
ground and screened material did not
display the mechanical uniformity of
the natural product. Although it did
not show excessive porosity, it con-
tained many granules either flat or
bearing pronounced promontories.
Roasting brought about considerable
shrinkage but no coking. Almost
without exception, the resulting ma-

terials had resistances lower than that
of transmitter carbon, in extreme
cases approaching that of graphite
The efficiencies of the materials pe
modulators were also lower than the
standard, approaching non-modula.
tion in materials with resistance as
low as graphite.

Ultimate success in the project
waits upon a satisfactory adjustment
of both the chemical and the physical
properties of the final material. The
results already obtained indicate that
the chemical adjustment depends
largely upon the final treatment,
where temperature is the most im-
portant factor provided the period of
its application is sufficiently long. In
determining the physical character-
istics, both the preliminary and final
treatments are important, the former
considerably influencing fracture and
binding.

It seems probable that further ex-
perimentation, varying the many de-
termining factors, would ultimately
reveal a procedure for producing
anthracite at least as good as the
natural. Necessarily empirical and
lengthy, such experimentation ap-
pears at present unjustified since the
condition of natural beds of anthra-
cite today suggests no pressing need
for substitutes. The experiments so
far performed have broken the way
to filling the need when it arises, and
have meanwhile contributed toward
a better understanding of the funda-
mental principles of coal formation
and of how to select the best anthra-
cite for manufacture into transmitter
carbon.

D. K. Martin had been actively in-
terested and engaged in radio work before
getting his B.S. degree, in 1916, from
the Polytechnic College of Engineering
at Oakland, California. Shortly after
graduation he went to Alaska for the
Alaska Packers Association where he was
occupied with the installation and opera-
tion of their radio facilities. During the
World War he was an En-
sign in the United States
Navy, spending much of
his time as radio instructor.
In 1919 he joined the Ameri-
can Telephone and Tele-
graph Company where he
continued his radio work in
the Department of Devel-
opment and Research.
Transferring to the Lab-
oratories in 1928 he engaged
in the development of air-
craft radio apparatus, and
at present he is supervisor
of the group which is hand-
ling radio transmission

W. C. Tinus began his radio career
with an amateur radio station immedi-
ately after the war. He was later with
several of the early broadcasting stations
in the southwest and also served at sea
as a radio operator. He received the B.S.
degree in Electrical Engineering from
Texas Agricultural and Me-
chanical College in 1928
and then joined the Tech-
nical Staff of these Lab-
oratories. Since that time
he has been concerned with
the development of airplane
radio equipment and with
its application to the rapid-
ly growing commercial air-
lines throughout the United
States.

H. B. FiscuHer received a
B.S. degree in Electrical
Engineering from the Uni-
versity of Wisconsin in 1924

and immediately joined the Technical
Staff of the Laboratories. With the radio
department he first worked on broadcast
receivers, but after the development of
aircraft communication apparatus was
started, he engaged in the design of avia-
tion receivers. At the present time, he is
in charge of the group developing radio
receivers for mobile systems.

The first call from an airplane in flight through the
Boston Marine Radiotelephone system was made on
Friday, March 10, when Capt. A. R. Brooks, piloting the
Laboratories’ trimotor plane, talked with the skipper of

Georges Bank, about 200 miles offshore. The call passed
through the Mendham laboratory and land lines via
Boston to the marine station of the New England Tele-
phone and Telegraph Company at Green Harbor, Mass.

Bell Laboratories Record 1933-05: Vol 11 Iss 9