flat type.* These have proved
economical to manufacture; and ex-
perience extending over a quarter of a
century has testified to their satis-
factoriness in operation. During this
time, however, the telephone system
has changed, and upon relays there
have been imposed operations more
complicated and critical than were in
mind at the time of their development.
Materials and manufacturing meth-
ods also have changed, so that all in
all it seemed desirable a few years ago
to undertake the design of a new all-
purpose relay. The vu-type relay—
shown in the photograph at the head
of this article—is the result.

Reliability in relay operation has
become of increasing importance in
recent years. The intricacies of the
dial systems require the operation of a
large number of relays on each call,
and for the satisfactory functioning of
the system many of these relays must
operate and release at just the right
time and without fail. The existing
types of relays do this, of course, with
what is really remarkable precision,
but occasionally a speck of dirt will
get between the contacts to prevent
their closing. Also the contacts may
“chatter” —rapidly opening and clos-
ing as a result either of the rebounding
of the armature when the relay is de-
energized, or of the independent vi-
bration of the springs. One of the
objectives of the new design, there-

fore, was to improve reliability by
making dirt particles less effective,
and by reducing the tendency to
chatter. Another objective was to
secure a greater number of contact
springs per relay. A study indicated
that an increase from the twelve
springs of the present relay to twenty-
four would be satisfactory. To obtain
the full advantage of such an increase,
however, the gain in number of con-
tacts must not be offset by a cor-
responding increase in size or in
energy required. In other words, more
effective and efficient use of materials
was sought.

An increased number of contacts
requires a larger magnetic flux; and if
the energy consumed by the winding
is not to be increased, this flux can
most effectively be secured by pro-
viding a magnetic path of lower re-
luctance. Such an improved magnetic
path was secured, first, by using a
larger core of circular cross-section,
which provides the greatest flux for a
given length of wire; and second, by
reducing the air-gap reluctance and
making more advantageous use of it.
In both the E- and r-type relays,
which are of the flat type, a u-shaped
armature is hinged to the yoke at the
rear of the core by a piece of thin
magnetic iron, as shown in Figure 1.
The spring hinge is so thin that only
a small part of the flux can be carried
by it; the rest of the flux passes
through air, which introduces into the
circuit an additional reluctance that
serves no useful purpose. The gap
at the hinge is kept as small as possi-
ble, of course, but it must be large
enough to permit free movement of
the armature after allowing for un-
avoidable unevenness at the hinge.

The armature of the new relay is of
the same u shape as that of the
R-type relay, but overlaps the end

yoke, instead of being hinged to it, and
is held loosely in place by a pin in each
arm of the vu. The construction is
shown in Figure 2. In its unoperated
position, the armature pivots on the
front edge of the rear bracket—
leaving a long wedge-shaped air gap
between this edge and the rear end of
the armature. As the armature closes,
this wedge-shaped air gap narrows
down until, in the operated condition
of the armature, it almost disappears.
This form of mounting tends to intro-
duce only a small reluctance, which is
made still smaller by the compara-
tively large cross-sectional area of the
hinge gap as compared with former
relays. Moreover, the flux in this gap
performs a useful function, since the
resulting force of attraction on the
rear end of the armature tends to
rotate the armature around the pivot-
ing point in the same direction as does
the relatively greater pull at the gap
at the front end. The core is milled
flat at its rear where it fastens to the
cross yoke, and also at its front to
form a broad flat surface for the arma-
ture. This front gap is limited in
width by the diameter of the core, but
it is made long enough to give a large
area and thus a low reluctance.

How effective this construction is
in increasing the pull is shown by
Figure 3, which gives a typical rela-
tionship between the force which the
armature can develop, and the power
required to maintain this force, for
both the r- and u-type relays. The pull
provided for six make contacts is
somewhat greater for the u-type relay
than for the R—more contact pressure
being provided—but the power re-
quired is only about a fifth. The
R type would be incapable of closing
12 make contacts, while the vu type
closes them with less than half the
power required by the r type.

Besides contributing to the pull of
the relay, this construction tends to
reduce the likelihood of chatter caused
by the armature rebounding after it
has opened. With the reed-hinged

armature of former relays, the full
force of the rebound occurs at the
front end of the relay where the
tendency to close the contacts is
great. With the u-type relay, how-
ever, the rebound divides between the
front gap and the loosely mounted
pivoting point farther back; and the
net consequence at the contacts is far
less than if the armature were free to
recoil only at the front.

Of at least equal importance is the
chatter caused by vibration of the
springs. Considerable study, both
theoretical and experimental, was
necessary to determine the nature and
causes of this vibration and the best
method of its elimination. It was
finally found that by properly propor-
tioning the dimensions of both the
stationary and the movable springs
this form of chatter could be elimi-
nated as a source of serious trouble.

The other major improvement in-
corporated in this new relay is the use
of twin contacts. Each contact spring
Carries two separate contacts in
parallel, so that even though one of
them should be held open by a speck
of dust, the other would close. Ags
already noted, the failure of a relay to
make contact is of comparatively rare
occurrence, but the provision of
double contacts very greatly decreases
this likelihood. The probability of a
random failure when two parallel
contacts are employed is the square
of the probability for a single contact.
If, for example, a single contact fails
to make once in ten thousand times,
then a pair of contacts in parallel
under similar conditions will fail only
once in a hundred million times.

With these various improvements
a relay has been made available that,
besides being able with less power to
close twice as many contacts as the
R type, is practically free from chatter
and from contact failure. In designing
the vu-type relay every advantage
has been taken of past experience and

of recent developments, not only to
make it more effective but also to
keep its manufacturing cost low.
With this in view a striking change
was incorporated in the method of
winding. In previous relays the front
and rear of the core are larger than the
section on which the winding is
placed, and the wire for the coil is
wound in place on the core. With the
new relay the core has been made of
the same diameter throughout its
length, so that it 1s possible to wind
the coils separately and then slip
them over the cores. This change in
the core was adopted largely to take
advantage of a method of winding
coils in multiple which was developed
by the Western Electric Company.
On a single arbor, as shown in Figure
4, seven coils are wound at the same
time from seven spools of wire. Be-
tween adjacent windings are slight
separations so that they may be cut
apart when completed. Between suc-
cessive layers of wire are very thin
layers of cellulose acetate sheet, which
run the full length of the arbor and
hold the wire in place.

The collection of spring contacts
consists of stationary and movable
springs, separated in their mounting
by strips of insulation. The stationary
ones are considerably thicker than the
movable, and do not bend appreciably
as the relay operates. The movable
springs are controlled by the armature
through insulating rods, which are
fastened to alternate springs and pass
through openings in the stationary
springs. The general appearance of a
spring assembly for twelve make con-
tacts, which is the full complement of
a relay, is shown on page 300. Each
stationary spring is a single piece
with the two contacts near the ends.
The movable springs are forked a
little beyond the point of attachment

to the operating rod, with each branch
carrying one contact. This gives com-
parative independence between the
two contacts of a pair, so that if one
is held open the other will still be free
to make contact. The soldering termi-
nals of these springs fan out at the
rear in the usual manner. For the
windings, soldering terminals are simi-
larly arranged, but they are also
extended toward the front of the

relay. This arrangement gives access
to the windings when testing from
the front.

The headpiece illustrates a relay
provided only with make contacts
but, as with most relays, other combi-
nations may be built up, such as
break-contacts, make-before-break,
break-before-make, or make-before-
make; several hundred combinations
may be obtained. For the windings
also there is a large number of pos-
sible ratings depending on the voltage
of the circuit in which the relay is to
operate and on other circuit charac-

teristics or on the type of
operation required. Other
optional features are copper
and aluminum sleeves which
are slipped over the core
when slower operation is
desired, and a split perm-
alloy sleeve to give the
winding a high impedance
to voice frequencies. The
cores are usually of mag-
netic iron, but permalloy
may be used when condi-
tions warrant it.

Because relays are used
in large numbers, their
space requirement becomes
of considerable importance.
The vu type requires verti-
cally the same space as the
flat relay; and horizontally,
space determined by its
number of springs. When
equipped to capacity, it has
a spring-pair density in the
plane of the mounting rack
of 4.2 pairs per square inch,
while the greatest density
before, which was with the
R type, was 3.4 pairs.

In the development of
the relay, many of the
mechanical and magnetic
problems have had to be
studied from both their the-
oretical and experimental
sides, and contributions
to the completed apparatus
have come from Labora-
tories specialists in many
different fields. Such mod-
ern research tools as the
high-speed motion-picture
camera and the string oscil-
lograph,* arranged to give
simultaneous electrical and
mechanical records, have

been indispensable in the
extensive tests that followed
the original design work. It
seems fair to say that only
through the insight afforded
by these instruments was
it possible to recognize the
true nature of many of the
design problems; and onl
through their use did the
final design come to so suc-
cessful a conclusion.

Development of this re-
lay had its inception in the
need for more contacts and
for lower battery drain; but
in the course of its design it
was found possible to add
many other desirable fea-
tures, such as twin contacts
and freedom from chatter.
As a result, it has become of
much wider utility than
was originally anticipated.
It seems not unlikely that
just as the need for new
switching mechanisms led
to its development, so its
new and better features
will open an avenue to
further progress in_ the
switching art. The 755A
PBX, which will be de-
scribed in a forthcoming
issue, introduced the new
design and was the first of
many systems to utilize it.
The most extensive use of
this relay is in the cross-
bar system, the first instal-
lation of which has been
made at the Troy Avenue
Office in Brooklyn.

Fig. 4—For the U-type relays
seven separate windings are
wound on a single arbor and
then cut off and assembled over
the core

positive electrode in a
vacuum tube, other electrons are
given off. These are called secondary
electrons. In ordinary vacuum tube
applications an effort is made to
reduce the number of secondary
electrons or to return them to the
electrode from which they came. If
not thus controlled, these secondaries
may introduce undesirable modifica-
tions in the tube characteristics. In
recent years a number of workers in
the vacuum tube field have endeav-
ored to apply this phenomenon to use-
ful purposes and as a result of their
efforts various devices known as elec-
tron multipliers have appeared. These
devices seemed to have potential
value in the field of applied electronics
and work has accordingly been under-
taken by the Laboratories with the
object of evaluating their worth and
of developing electron multipliers
that may be adapted to our purposes.

The number of secondary electrons
leaving a surface struck by a stream
of electrons is proportional to the
number of primary electrons striking
it. The proportionality factor varies
both with the potential drop through
which the primary electrons fall
before striking, and with the nature
of the emitting surface. If there is to
be amplification this proportionality
factor must be greater than one, so
that both the potential drop and the
nature of the electrode surface are
important factors in the design of the
multipliers. With silver-oxygen-cesi-
um surfaces, similar to those used in
modern photoelectric cells, and a
potential drop of 100 volts, it is
possible to get a proportionality factor
of four. Under these conditions the
electron stream is multiplied fourfold
every time it strikes an emitting sur-
face, and by arranging a number of
such stages in series, a considerable
amplification can be obtained.

thus consists of a photoelectric surface
on which light falls to produce the
initial stream of electrons, and a
series of plates, each at a higher
potential than the preceding one, to
provide successive stages of ampli-
fication. The operation of such a
multiplier is illustrated schematically
in Figure 1. Light falls on the photo-
cathode, 0, and produces an electron
current 1. This current falls through

Fig. 1—Many of the previous electron
multipliers have used a series of metal
plates at successively higher potentials, and
have employed a magnetic field to guide the
electrons from one plate to the next

the potential drop v, which is the
potential difference between plate 1
and the photo-cathode, and on strik-
ing plate 1 produces mi secondary
electrons, where m is the proportion-
ality factor. These electrons falling
through a similar potential drop to
plate 2 produce a group of secondary
electrons m times as great, so that the
current leaving plate 2 will be m?1.
This process is continuous over the
entire series of plates, and results in
an output current of m1, where k
is a number of plates or stages. The
last plate acts as a collector and is
connected to the output circuit.

If such a multi-stage multiplier is
to function satisfactorily, the electron
leaving one plate must be guided by
electric or magnetic fields so that they
will all strike the next plate within a
small area, which should preferably
become smaller from plate to plate.
If the area within which the electrons

struck increased from plate to plate,
some of the electrons would sooner or
later fail to reach the next plate, and
there would be a loss in amplification
due to diffusion. If the electrons can
be made to fall within smaller and
smaller areas in going from plate to
plate, there will be no diffusion loss,
Moreover, there should be a strong
field away from each plate, or space
charge will form, and the secondaries
will not all leave.

If the electrons are to follow a con-
verging path in a multiplier with
magnetic focussing, there must be an
accurate balance between the electric
and magnetic fields. If the voltage
drops a little while the magnetic field
remains constant, the electrons will
go to the wrong place, and multipli-
cation will be markedly lowered.

Fig. 2—Diagrammatic arrangements of
plates of one of the recently developed elec-
tron multipliers

Because of this critical balance that
must be maintained between the two
fields, and because the presence of
magnetic fields may in itself be ob-
jectionable, it was desirable to develop
a multiplier that did not require a
magnetic field. Although some of the
early multipliers used only an electric

field, they all had low efficiency
because of weak fields away from the
plate, and poor convergence due to
inadequate control of the paths of
the electrons.

To design a satisfactory multi-
plier, the paths of the electrons must
be accurately known, but in struc-
tures as complicated as multipliers,
computation of the paths is so difficult
as to be almost out of the question.
Fortunately, however, where the field
is only two dimensional it is possible
to establish an analogy between the
path of an electron in an electric field
and the path of a ball on a tightly
stretched horizontal membrane that
is deflected slightly in certain places.
Such a two-dimensional field will
exist wherever a group of electrodes
form equipotential surfaces extending
parallel for a long distance in one
direction. In Figure 3, for example, a
set of such equipotential surfaces is
shown, which extend parallel for a
long distance in the direction of the
z axis. With voltages on these plates
as indicated the electric field would be
two dimensional; the field at any
point between the two rows of plates
would have an x and a y component
but no z component. An electron
multiplier employing only an electric
field to guide the electrons is essen-
tially this sort of a thing, the electrons
passing successively from plate to
plate in order of increasing voltage.

A mechanical analogy of such a
two-dimensional field can be formed
by stretching a rubber sheet tightly
in a horizontal plane with sections of
it depressed to form levels of different
gravitationed potential. Small balls
rolling along such a surface under the
influence of the difference in gravi-
tationed potential, or height, would
follow paths exactly like those of
electrons in a similar distribution of

electric potentials. In this analogy the
charge of the electrons is proportional
to the weight of the ball, the mass of
the electron to the mass of the ball,
the electric potential difference be-
tween plates to the difference in

Fig. 3—A two-dimensional field exists be-
tween a series of equipotential plates that
are very long compared to their width

height between successive level areas
of the rubber diaphragm, and the
electric field strength to the slope of
the rubber surface, while the equi-
potential electrodes correspond to the
level areas of the rubber surface.

To determine the proper arrange-
ment of electrodes to cause the elec-
trons to pass through the multiplier
in the desired manner, such mechani-
cal analogs have been constructed.
The equipment in the accompanying
photographs was constructed under
the direction of W. Shockley. A sheet
of rubber is stretched tightly to a
rectangular frame of angle iron, which
is held horizontal but arranged so
that it can be raised or lowered with

Fig. 4—Side view of the mechanical analog showing the
stretched rubber sheet and the metal vanes that press it down
from above so that it lies across the blocks beneath

respect to a rigid horizontal surface
beneath it. Wood blocks, with accu-
rately leveled top edges, are set up on
the lower horizontal surface to form
surfaces of constant height when the
rubber sheet is lowered to rest on top
of them. To force the rubber to lie
along surfaces, metal
vanes with horizontal
lower edges are ar-
ranged to press down
the rubber sheet so
that it lies across the
tops of the wooden
blocks beneath. To
make them show up
more clearly in the
photograph, the equi-
potential surfaces are
marked by straight
chalk lines, and two
possible paths of balls
rolling from one sur-
face to the next lower
one are also marked
clearly with chalk. The

investigation is to try
various sizes, shapes,
and positions of blocks
until balls rolled from
one level surface to the
next lower one tend to
converge smaller
areas in each succes-
sive stage. A ball
started any place over
a wide area of plate 1,
for example, should
roll so as to strike the
next lower plate in the
corresponding _ region
but over a narrower
area. The arrangement
shown in the photo-
graphs is that actually
employed for one of
the recently developed
multipliers, and the convergence of
the balls under actual rolling condi-
tions is shown in Figure 6.

The arrangement of the blocks, and
of the plates of the multiplier based
on it, is shown in Figure 2. The con-
vergence of this multiplier is very

good, and in addition there is a strong
field away from all portions of the

lates where the electrons strike. The
effect of initial velocities in the paths
of the electrons was investigated by
starting the balls on the rubber model
with a slight initial velocity, and was
found not to be particularly serious.

A number of multi-
pliers of this type have
been built, and one of
them is shown in the
photograph at the
head of this article.
The photo-cathode
has an effective sur-
face considerably
larger than that of the
multiplying plates, of
which there are six
between the photo-
electric surface and the
anode, or collector. All
of the plates are long
compared to their
width, in a direction at
right angles to the
plane of travel of the
electrons, and the ends
of the plates are bent
in toward the center to keep the
electrons from drifting out. The
problem of activating the multiplier
so as to get a good photoelectric
response and at the same time good
secondary emission was solved by
M. S. Glass, and at 750 volts, overall,
this multiplier has an output of thirty
milliamperes per lumen, which is over
a thousand times greater than that of
a vacuum photocell. The safe output
current is of the order of a few milli-
amperes, and the collector voltage
may swing as much as 70 volts each
side of the mean value without ser-
lously affecting the operation of the
multiplier. The output resistance,
which varies inversely with the out-

The electron multiplier has a num-
ber of advantages as compared with a
photoelectric cell and an equivalent
vacuum-tube amplifier. In the first
place it is much smaller, not being
much larger than the photoelectric

Fig. 6—Top view of analog showing two balls rolling from
widely separated points at one potential level and converging
as they approach the next lower level

cell alone. Further, experience has
shown that for high-frequency work,
the electron multiplier is less noisy.

The multiplier is also practically
non-microphonic; the noise in the
output is little higher than the un-
avoidable “‘shot” noise of the photo-
electric cell multiplied by the gain.
Still another advantage is that its
amplification is practically constant
at all frequencies up to several mega-
cycles. The emission of the secondary
electrons is so nearly simultaneous
with the impact of the primaries that
the multiplying action faithfully re-
produces the very rapid changes in
current necessary to transmit these
very high frequencies.

N many of their applications in the
Bell System, relays are required to
act as rapidly as possible. They

should operate promptly when volt-
age is applied to them, and release
promptly when the circuit to their
winding is opened. There are numer-
ous applications, however, where it is
necessary for the relay to remain
operated for an interval after the
circuit to its winding is opened, and
this release interval is often specified
within very close limits. Precise be-
havior of this kind is not easy to
secure; only careful design and accu-
rate control of manufacture make it
possible. Relays that are most eco-
nomical for general uses will not meet
the very exacting requirements laid
down to secure a precise slow-release
period. When the general utility v-
type relay was developed, it seemed
desirable, therefore, to develop at the
same time a slow-release relay that so
far as possible would use the same
parts, manufacturing tools, and proc-
esses. The result was the y-type

The need for dependable slow-
release relays can be well illustrated
by one of their applications in the
panel system. A group of relays in the
sender records each digit dialed, and it
is necessary to switch the dialing cir-
cuit from one group to the next after
each digit of the called number. As
the dial returns after having been
pulled around to one of the digits, it
opens and closes the circuit in rapid
succession to indicate the digit dialed.
Thus in dialing 2 the circuit will be
opened and closed twice, and in dial-
ing 4 the circuit will be opened and
closed four times. Immediately after
its return, the dial is pulled around
for the next digit, and during this
short interval between digits the cir-
cuit must be switched to the next
group of recording relays. This 1s
accomplished through a slow-release
relay that does not release during the
short open periods of each digit, but
does in the slightly longer period be-

tween digits. It must do this regard-
less of the commercial range in dial
speeds, of voltages and line variations,
and in the speed with which the dial
js pulled around. In the latest type
sender the actual requirements for
the y-type relay are that it shall re-
lease in not less than 0.080 second nor
in more than 0.120 second. The ordi-
nary relay, however, releases in from
0.005 to 0.015 second. In designing a
slow-release relay, therefore, there are
two objectives sought: first to provide
the slow-release action, and second to
provide for an accurately controlled
release time.

In the case of an ordinary relay
when the winding circuit is opened
to release the relay the current de-
creases almost instantly to zero value
except for the duration of any arcing
at the opening contacts. The relay
continues to hold in the operated
position, however, because a consider-
able portion of the flux in the mag-
netic circuit is maintained by eddy
currents which are induced in the iron
upon breaking of the winding circuit.
The effect of the eddy currents is more
prolonged if the air gaps in the mag-
netic circuit are small.
Tohastentheactionof

made of brass or other non-magnetic
material. In the y-type relay, however,
they are omitted so that when the
relay is operated, the armature is di-
rectly in contact with the core.

The omission of the stop discs re-
duces the current necessary to hold
the relay operated, and since the
release time is a function of the
difference between the maximum flux
and the flux at which the relay just
holds, there will be a longer time
between the opening of the circuit and
the release of the relay. To make a
further decrease in the reluctance of
the magnetic circuit, the cross-yoke
at the rear of the core of the y-type
relay and the hinge bracket on which
the rear ends of the armature rest are
made of magnetic iron instead of cold-
rolled steel, as are those that are used
in the u-type relay.

The major means of securing slow
release, however, is by retarding the
rate of decrease of flux in the core by
providing during the release period a
magnetizing force that tends to main-
tain the flux at its original value. This
is done by placing directly over the
core a sleeve of copper or other con-

comparable action on o 4
still other relays is
obtained also by means
of an adjustable screw

ducting material. As the flux tends to
decrease, when the winding circuit
is opened, a current is induced in this
sleeve which is in a direction tending
to maintain the flux. The lower the
resistance of the sleeve, the larger will
be the current that will flow, and the
less the rate of dissipation of energy
due to resistance. To provide control
by this means, three copper sleeves of
different thickness and one of, alumi-
num are employed. Aluminum is used
for the shortest release period because
of its high resistance. To obtain a high
enough resistance with copper, the
sleeve would be too thin for satis-
factory manufacturing conditions.
These sleeves provide four degrees
of delay that may be obtained with
the y-type relay. The actual release

Fig. 2—Simplified cross-section of the
magnetic circuit of the U-type relay

time, however, depends not only on
the resistance of the sleeve and on the
reluctance of the magnetic circuit,
but on the restoring force of the
springs tending to open the relay.
This, in turn, is a function of the
number of springs with which the
relay is equipped, so that the same
sleeve will give different release times
depending on the springs used. The
number of ampere turns required to
hold the relay operated also depends
on the spring load, so for each sleeve
a curve can be plotted showing the
relationship between release time and
the ampere turns required to hold the
relay operated. A set of such curves

for the four sleeves is given in Figure
1; the No. 1 curve is for the aluminum
sleeve and the other three are for
those of copper.

These copper sleeves provide the
most effective means of retarding the
decrease of flux in the core after the
winding circuit is opened. Any short-
circuited winding on the core, how-
ever, would provide the same action
only less effectively. Advantage is
taken of this fact in a few applica-
tions where the release time desired
falls between two of the curves of
Figure 1. Short-circuited windings are
designed to give the desired inter-
mediate release times.

After having decided on_ these
various features to secure a longer
release time, it is necessary to make
other changes to hold the desired time
within very close limits. The design
must provide not only that any one
relay will always release after the
desired interval within narrow limits,
but that all relays built for the same
release interval will meet their re-
quirements within similar limits.

The chief factor causing variations
in release time may be illustrated by
Figure 2, which is a simplified cross-
section of the magnetic structure of
the u-type relay. The contact surface
of the relay core and of the hinge
bracket are both flat, and so are the
two corresponding surfaces of the
armature. If the armature, core, and
hinge bracket could be _ perfectly

Fig. 3—Simplified cross-section of the
magnetic circuit of the Y-type relay

aligned, the area of contact, and thus
the contact reluctance, would be the
same under all conditions. Actually,
however, slight variations in the thick-
ness or position of the hinge bracket
or of the armature itself, cause the
armature to meet the pole face at a
small angle, which will vary with
different relays and with the same
relay at different times.

With the u-type relays variations
due to these causes at the front end of
the relay are reduced by the stop
discs, and the variations at the rear
end are not great enough to cause de-
viations that exceed the requirements.
With the y-type relay, however,
which does not employ the stop discs,
these variations would seriously im-
pair the accuracy of the release inter-
val. The variations are avoided,
however, by the construction shown
in Figure 3. A small spherical surface
is embossed on the front end of the
armature so that the armature always
meets the core in what is essentially a
point contact. The front end of the
hinge bracket is embossed to form a
cylindrical surface at right angles to
the axis of the armature; and so the
armature and hinge bracket always
meet in what is essentially a line con-
tact. Regardless of the alignment of
the armature, therefore, there is
always a point contact at the front
end and essentially a line contact at
the rear end.

The effects of these structural
differences on the reluctance are
shown in Figure 4 for relays with flat
surfaces and for those with the
spherical and cylindrical surfaces of
the y-type relay. With perfect align-
ment the reluctance of the flat sur-
faces is very low, but it increases
rapidly with an increase in the angle
between core and armature. The
reluctance of the rounded surfaces is

much higher for perfect alignment,
but it increases only very slightly as
the deviation in the alignment in-
creases, and thus gives the constancy
of reluctance particularly desirable
for a slow-release relay.

Fig. 4—Variations in reluctance of relay
with angle at which armature meets the core
for both flat and embossed pole pieces

ment—zero angular deviation—the
contact is spread over the entire
surface, and the reluctance as a result
is very low. When the armature meets
the pole face at an angle, however,
there is a wedge-shaped air gap
formed, and the greater the angle of
contact, the greater will be the width
—and thus the reluctance—of this
gap. With the spherical or cylindrical
surfaces, the contact is always in a
point or line with a narrow air gap at
the sides regardless of the angle with
which the armature meets the con-
tacting surfaces. The reluctance of
such a contact is higher than that of
two flat surfaces where the angle is
small, but remains essentially con-
stant, while that of the flat surfaces

Fig. s—Variations in release times for flat and rounded pole
surfaces. Shaded area shows variations for Y-type relays

A protective coating is plated on
the magnetic structure to prevent
corrosion of the iron, but since this
coating is non-magnetic, it introduces
what is effectively an air gap in the
magnetic circuit. Variations in the
thickness of this plated coat would
affect the release time by varying the
reluctance of the magnetic circuit.
The closeness with which the release
time must be held with the y-type
relay, therefore, requires that the
thickness of the plating be held within
very close limits; the actual range is
from 0.0003 to 0.0006 inch. Chromium
is used for the outer plating to pro-
vide a very hard surface to resist wear
at points of contact between the arma-
ture and core.

As a result of the consistency in
reluctance obtained by these various
means, the variations in release time
are considerably reduced. For the
y-type relay the variations in release
times are shown by the shaded area
of Figure 5. For a corresponding relay
with flat surfaces, the release time
might fall anywhere between the
curves A and B. With rounded sur-

Heretofore it has been
necessary to check the
operation of slow-re-
lease relays by actually
measuring the release
times, which is a rather slow and
expensive procedure. With the y-type
relay such measurements are un-
necessary; it is sufficient to measure
the holding current, because of the
close correspondence between holding
current and release time.

Except for these differences, the
y-type relay is the same as the u type,
and to the casual glance one could not
be told from the other. Moreover, the
copper sleeves designed for the y-type
relay may be employed with the
U type to secure somewhat slower
release, although the long release
obtained with the y-type relay cannot
be secured with the vu type, nor can
the accuracy of release time for the
relay be maintained.

As the u-type relay is intended to
be the general utility relay of the
future, replacing ultimately the & and
R types, so the y type will be the
slow-release relay of the future—
replacing the 149, 162, 178, and T
types. One of its first applications 1s
in the new 755A PBX, where it will
be used, in conjunction with other
relays, to simulate the action of a
ringing interrupter.

A movEL showing the general exterior
design of the Bell Telephone System
Building at the New York World’s Fair in
1939 has been approved, and construc-
tion is now under way. The building
will occupy a triangular plot of slightly
more than three acres immediately north
of the Theme Center; formal entrance is
through a pavilion at the point of the
plot adjacent to the Theme Plaza. In the
semi-circular court of this pavilion is a
sculptured group by Carl Milles. From
this point to the building proper a walk
leads through a grove of 150 mountain
pine trees, chosen because they allow
ample room for people to move about
under their spreading lower branches.
Several pools and fountains add to the

The other entrances to the building,
fronting on the two main avenues which
bound the plot, will be flanked by tall
vermilion panels which will be decorated,
respectively, by Hildreth Meiere and
Edward Trumbull. The mass of the
building rises to different heights to
house the varied exhibits that are now
being developed in Bell Telephone Lab-
oratories. Along the south side of the site
runs a decorative colonnade. Where the
axis of the Transportation of the Fair
meets this side of the plot, the building
wall presents a surface fifty feet high
which it is proposed to cover by a large
map showing cities in relief and animated
by changing lights to indicate the main
lines of telephonic communication around
the world. Above the large circular wing

munication surmounts the _ building,
which is lighted in such a manner to form
a distinctive night display.

This exhibit of the American Telephone
and Telegraph Company, like its other
recent exhibits, is being carried out
under the direction of Vice Presidents
Arthur W. Page and Frank B. Jewett.
To the Laboratories there was dele-
gated the conception and execution of the
exhibits; responsible for these develop-
ments are John Mills and M. B. Long.

The building, the architect’s model of
which is shown on page 299 and also on
the preceding page, is a design of Voor-
hees, Gmelin and Walker. The interior,
including the display of exhibits and the

W. H. Harrison, a member of the Board of Directors
of these Laboratories, has been elected to succeed
Bancroft Gherardi, who retired on the thirtieth of
April, as Vice President and Chief Engineer of
the American Telephone and Telegraph Company

decorations, is being designed by Henry
Dreyfuss. The building itself will be con-
structed by Marc Eidlitz and Son.

When the building has been com-
pleted and its exhibits installed the opera-
tion and management will be turned over
to the New York Telephone Company,
under the general supervision of its Vice
President Carl Whitmore and the direct
charge of Thomas W. Williams.

Bancroft Gherardi, a Director of these
Laboratories from 1925 to 1937, retired on
April 30 from the office of Vice President
and Chief Engineer of the American
Telephone and Telegraph Company. So
ended a distinguished career in the Bell
System, whose forty-three years
have seen almost the entire de-
velopment of the telephonic art.
Under the continuous leadership
of Mr. Gherardi, the Department
of Operation and Engineering of
the American company has be-
come a vital force in the progress
of the Bell System. Throughout
the period of rapid evolution in
the communication art, nothing
has stood out more than the suc-
cessful organization of the tech-
nical forces in all the branches of
business activity involved. In the
direction of this organization
work the record of Mr. Gherardi
has been outstanding.

In 1895, with the degrees of
B.S. from the Brooklyn Poly-
technic Institute and M.E. and
M.M.E. from Cornell University,
Bancroft Gherardi started his
telephone career testing some of
the relatively few telephone
cables in the area now operated
by the New York Telephone
Company. Within eleven years,
during which time he gained con-
siderable traffic as well as plant
engineering experience, be-
came assistant chief engineer of
both the New York and New
Jersey companies. In 1907, he

joined the American
Telephone and Tele-
graph Company as eng}-
neer of building and
central office equipment.
Three years later he be-
came engineer of plant in
charge of plant develop-
ment and standardiza-
tion for the Bell System.
When the Engineering
Department was reor-
ganized in I919, Mr.
Gherardi became Vice
President and Chief En-
gineer in charge of the
Department of Opera-
tion and Engineering.

In technical associa-
tion work and in co-
operating with related
industries, Mr. Gherardi
has always been very
active. He led in the for-
mation of the Joint Gen-
eral Committee of the
Edison Electric Institute
and the Bell System and
has served as Bell Sys-
tem Chairman. He has
also been Bell System
Chairman of the Joint
General Committee of the Association of
American Railroads and the Bell System
concerned with somewhat similar prob-
lems. In the American Institute of Elec-
trical Engineers he has been a manager,
Vice President, and was President during
the 1927-1928 year. He has served on such
committees as the Executive, Edison
Medal, Finance, Headquarters, Public
Policy, Research and Constitution Re-
vision. He has represented the Institute
upon the Board of Trustees of the
United Engineering Trustees, the Library
Board, the National Research Council,
the United States National Committee of
the International Electrotechnical Com-
mission, the John Fritz Medal Board of
Award, and other bodies. He has also
been associated with other activities,
such as Trustee of Cornell University,

President of the American Standards As-
sociation, representative of the Bell
System in charge of relations with the
International Advisory Committee on
Telephony and chairman of the Engi-
neering Section of the National Academy
of Sciences.

Some of the honors conferred on Mr.
Gherardi are honorary degrees of Doctor
of Engineering from the Brooklyn Poly-
technic Institute and from the Worcester
Polytechnic Institute, the emblem of the
Fourth Order of the Rising Sun from
Japan and the A.I.E.E. Edison Medal.

Improved methods of manufacture at
Hawthorne, illustrative of the system
of product shop organization, was the

subject of an interesting talk given on
April 7 by David Levinger, Engineer of
manufacture of the Western Electric
Company, before over 500 members of the
technical staff of the Apparatus Develop-
ment Department. Mr. Levinger de-
scribed the principles and advantages of
the new system and illustrated his talk
with lantern slides of the plant set-up
and motion pictures of processes of manu-
facture of station apparatus. The talk
was arranged by a committee consisting
of O. A. Shann, chairman, P. S. Darnell,
J. B. Dixon, E. C. Edwards, R. C. Koer-
nig, G. Puller, W. H. Sellew, H. D.
Wilson, and J. M. Wilson.

THE TRIAL of the Type-K carrier tele-
phone systems which were installed be-
tween Toledo, Ohio, and South Bend,
Indiana, during May and June, 1937, has
been completed. Additional equipment is
now being installed between Toledo and
Detroit and it is expected that the com-
plete project which will provide three
systems between South Bend and Detroit
and six system between Toledo and
Detroit will be placed in service within
a few months. Each system provides
twelve telephone circuits. An article
describing the Type-K carrier system was
published in the April issue of the Recorp.

During the trial period, the operation
of the systems was carefully studied. A
considerable number of Laboratories’
engineers were involved in these tests,
and in addition active codperation was
obtained from the Long Lines Depart-
ment and the Department of Operation
and Engineering of the American Tele-
phone and Telegraph Company. The per-
formance of all parts of the circuit was
checked in detail, and the cable pairs
balanced to reduce crosstalk. Measure-
ments were made of the overall perform-
ance, including such items as transmission
regulation, equalization, noise, crosstalk,
overall stability, and reliability of opera-
tion. Tests were made of the echo, singing,
and transient characteristics of these cir-

cuits, especially as affected by switched or
permanent connections to other types of
facilities. All troubles experienced were
investigated, improvements in design
made where necessary, and testing and
adjusting methods worked out which
will facilitate maintenance of the system.
Much of the information obtained in this
trial is of value in applying the Type-K
system over longer distances than the 150
miles between Toledo and South Bend.

During the month of December, 1937,
ten circuits on each of the two trial sys-
tems were placed in regular commercial
service for a period of several weeks. They
were used to provide additional circuits
required for the Christmas and New Year
holidays, and many were extended by
other facilities to form circuits between
widely separated points. During the
service period, observations were made by
special service observers. No troubles
were experienced attributable to the
Type-K systems, and the performance
was entirely satisfactory. This repre-
sented the first commercial operation of
systems comprising as many as twelve
carrier channels each on existing toll
cable pairs.

K. K. Darrow discussed neutrons at
the March 7 meeting of the Colloquium.
He described the career of the free neu-
tron from its beginning in a process of
transmutation to its end in another such
process. Of the neutron-releasing proc-
esses, several were singled out for presen-
tation because of one feature or another,
ending with the one from which the most
accurate currently accepted value of neu-
tron-mass is derived. Of the neutron-
capturing processes, several were singled
out because of particular features, such
as the formation of free alpha-particles,
the formation of radioactive substances or
the generation of helium in quantities
large enough to be detected by micro-
chemical methods. The deflections and
scatterings, with and without energy loss,
to which a neutron is subject as it passes
close by nuclei between its release and its

Precise measure-
ment is basic to all
science; hence the
endless variety of
measurements made
in Bell Telephone
Laboratories. For
many projects no
standardized meas-
uring apparatus ts
available; hence it
must be built to
order.

II
Measuring the sepa-
ration between pole
pieces and dia-
phragm of an audi-

Measurement of the
thickness under com-
pression of a paper
damping-ring book
for a transmitter

ultimate capture, were discussed to-
gether with the observations on the
nuclei which recoil from these impacts.

At the March 21 meeting, Dr. D. W.
Bronk, Director of the Eldridge Reeves
ohnson Foundation for Medical Physics,
the School of Medicine of the University
of Pennsylvania, spoke on The Physical
Basis of Sensory Mechanisms. According
to Dr. Bronk the contacts which a living
organism maintains with its environment
through the sense organs depends upon
the reactions of the organized matter in
nerve tissue. The existence of living mat-
ter in such a state of organization de-
pends upon a continual expenditure of
energy. Alterations in the energy supply
may under appropriate conditions initiate
a series of propagated waves of activity
passing from the sense organ to the brain.
In terms of these series of events it is
possible to analyze many of the compli-
cated processes set up in the brain and to
obtain a clearer understanding of the
fundamental physico-chemical properties
of the sensory receptors.

F. S. Goucuer presented a paper en-
titled The Physics of Microphonic Con-
tacts, before the Metropolitan section of
the American Physical Society at a meet-
ing held at Columbia University on
March 25. Dr. Goucher traced the evo-
lution of the carbon microphone, showing
slides of early models and demonstrating
the action of some of these with the aid of
a magnetic tape recorder. He discussed
the first quantitative theories as pre-
sented by Pedersen in 1916 and by Gray
in 1920 and then described the research
work carried on more recently by the
Laboratories with special emphasis on the
elastic behavior of microphone contacts.

Dr. Bucktey attended part of the
General Plant Managers’ Conference
held at Virginia Beach during the week of
March 21. This conference was arranged
by the Department of Operation and
Engineering of the A. T. and T. Company.

W. Witson has been appointed chair-
man of the Papers Committee of the
Institute of Radio Engineers and a mem-
ber of the Board. of Editors and of the
Standards Committee.

L. Monramar attended a meeting of
the National Executive Committee of the
Telephone Pioneers of America.

THE MATHEMATICS DEPARTMENT of
Princeton University is sponsoring, dur-
ing the current academic year, a series of .
lectures by men in industry to give the
undergraduate students some idea of the
mathematics plays in industrial
work. On March 14, W. A. Shewhart
spoke on Some New Fields of Application
of Mathematics in Industry, setting forth
the way in which statistical method plus
mass production equals a new tool of re-
search, applicable in every stage of the
production process from raw materials to
finished product.

R. H. Couey lectured on wood preser-
vation before an advanced short course
for tree wardens and town foresters. This
course was sponsored by the Massachu-
setts State College at Northampton.

Durinc Marcu various problems were
discussed by members of the Labora-
tories with members of the manufac-
turing organization at Hawthorne. Prob-
lems of switchboard cord developments
were discussed by C. H. Greenall and R.T.
Staples; lineman’s handsets, station hand-
sets and repeater headbands by A. F.
Bennett; handsets by J. Dalton and
W. G. Turnbull, Jr.; central office alarms
for dial offices by N. H. Thorn; crossbar
equipment by D. H. Wetherell; general
circuit and central-office developments
by W. L. Filer and J. Irish; and step-by-
step equipment by C. G. McCormick. Mr.
McCormick also visited central offices in
Minneapolis and St. Paul to observe the
performance of this type of equipment.

G. R. Lum visited the Ingraham Clock
Company at Bristol, Connecticut, to
discuss loudspeaker cabinet design.

C. F. Wigsuscu was at Virginia Beach
from March 18 to 22 to assist in arrang-
ing equipment shown at the General
Plant Managers’ Conference.

M. W. Lane of Hawthorne visited the
Laboratories the week ending March 11
and discussed problems associated with
the manufacture of the combined set.
J. T. Kane of Hawthorne also visited the
Laboratories the week ending April 1 to
confer on similar problems.

W. A. Evans and R. Burns attended
committee meetings of the American
Society for Testing Materials held at
Philadelphia. Later meetings held at
Rochester were attended by J. R.
Townsend, Mr. Evans, Mr. Burns, C. H.
Greenall, H. E. Haring, H. G. Arlt, and
W. J. Clarke. Mr. Greenall was elected
chairman of the committee on Copper and
Copper Alloys, Cast and Wrought.

J. R. Townsenp, on April 8, spoke be-
fore the Hawthorne Science Club on New
Materials and Procedures in Telephony.

W. W. Werrinc visited Schenectady
to discuss, with Professor Sayre of Union
College, plans for the forthcoming sym-
posium on impact testing to be held
under the auspices of the American
Society for Testing Materials.

AT THE REGULAR meeting of the Radio
Colloquium held at the Holmdel labora-
tory, A. P. King discussed the latest de-
velopments in wave-guide transmission.

W. L. Brack inspected speech-input
equipment installations at WJR, Detroit
and WBNS, Columbus.

E. G. Fracker and H. C. Rusty con.
ferred with members of the Ingraham
Clock Company at Bristol, Connecticut,
on March 7.

A. F. Price and A. H. super.
vised the installation of flight deck an-
nouncing equipment on the U.S.S. Enter
prise at Newport News during the past
month. Mr. Price also inspected the
electric clock system of the U.S.S. Phila.
delphia at the Philadelphia Navy Yard
on March 14.

J. D. Kvetnxaur and H. C. Curt
visited the Navy proving grounds at
Dahlgren, Virginia, during March in con-
nection with the installation and testing
of loudspeaker systems.

AN INVESTIGATION of vacuum-tube per-
formances in telephone repeater circuits
made it necessary for J. R. Wilson and
H. A. Pidgeon to visit Atlanta, Washing-
ton and Princeton.

V. L. Roncei and Mr. Wilson discussed
glass problems with engineers of the
General Electric Company, Schenectady.

F. S. Goucher, assisted by F. R. Haynes, demonstrating the behavior of the earliest forms
of telephone transmitter as compared with some of the later types

A. E. Rueuwve and E. K. Jaycox at-
tended a meeting of the Intersociety
Color Council at the Electrical Testing
Laboratories held in New York.

R. M. Burns attended a conference
held at Rochester, which was called to
consider the feasibility of organizing the
American Joint Committee on Corrosion.
He also addressed the Southern Tier
Chapter of the American Society for
Metals at Waverly, New York, on
Protection of Metals from Corrosion.

Harvey FLETCHER is the author of an
article entitled Loudness, Masking and
Their Relation to the Hearing Process and
the Problem of Noise Measurement, pub-
lished in the April issue of The Fournal of
the Acoustical Soctety.

W. SHocKLEy spoke before a group at
the Massachusetts Institute of Tech-
nology on April 6 on Order and Disorder in
Alloys. On the next day he also spoke
before another group of the Institute on
the subject Elementary Notions About
Electrons in Atoms and Crystals.

AN arTIcLeE entitled 4 Theory of Noise
for Electron Multipliers, by W. Shockley
and J. R. Pierce, was published in the
March issue of the Proceedings of the
Institute of Radio Engineers.

A. E. Bowen, on March 3, discussed
Wave Guide Transmission before the
Philadelphia Section of the Institute of
Radio Engineers.

G. C. SourHwortH discussed and
demonstrated the propagation of ultra-
high frequency radio waves through wave
guides before the Pittsburgh section of
the IL.R.E. on March 14.

K. K. Darrow spoke on modern de-
velopments in the field of radioactivity on
March 10 before the Rensselaer Poly-
technic Institute’s chapter of Sigma Xi.
On March 29, Dr. Darrow addressed the
Electrophysics Group of the A.I.E.E. on
Electrical Phenomena in Gases.

A. R. Kemp, Chairman of the Rubber
Division of the American Chemical
Society, has been appointed by President
Whitmore as representative of the Society
at the Rubber Technology Conference to
be held under the auspices of the Insti-

tution of the Rubber Industry in London,
England, May 23 to 25. As a member of
the Papers Committee of the Conference,
Mr. Kemp has been handling all of the
papers prepared by engineers in this
country. He will also present two papers

of the Central Office Switching Develop-
ment Department completed thirty years of
service in the Bell System on April 11

himself, entitled, Composition and Col-
loidal Properties of Balata Latex and
Dielectric Measurements in the Study of
Carbon Black and Zinc Oxide Dispersion in
Rubber with D. B. Herrmann as co-author
of the latter.

On March 28 and 29, Mr. Kemp at-
tended a meeting of the Rubber Division
of the American Chemical Society held
in Detroit.

R. L. Lunsrorp and J. T. Morrer, to-
gether with representatives of the General
Installation Engineer’s organization, in-
spected the 350-type unattended dial
office at Brewster, New York.

F. F. Stepert observed the perform-
ance of Diesel engines at Baldwin,
Pennsylvania.

J. L. Larew observed the operation of
the 410-A power plant in the Pennsy]l-
vania Railroad Offices at Harrisburg.

J. W. Scumiep and W. F. Simpson ap-
peared before the Court of Customs and
Patent Appeals in Washington in inter-
ference proceedings.

O. E. Rasmussen was at the Patent
Office in Washington appearing before
the Board of Appeals relative to an appli-
cation for patent.

Mr. Rasmussen completed twenty-
five years of service with the Bell System
on the twenty-first of April.

A coMPLETE installation of toll cable
equipped with telephone repeaters, sig-
naling and telegraph equipment of
Western Electric manufacture is now in
service on the Pennsylvania Railroad be-
tween New York, Philadelphia and
Harrisburg in connection with their elec-
trification project. The equipment on the
Philadelphia-Harrisburg route in-
spected recently by A. Kenner.

THE TRIAL of the initial installation of
the new Type-K carrier telephone system
on the Toledo-South Bend cable has been
completed. The following engineers, who
have been observing at the terminals and
intermediate points, have returned to
New York: C. A. Grierson, J. P. Kinzer,
W. D. Mischler, L. H. Schwartz and R. L.
Tambling. F. A. Brooks with S. Lewis,
C. Randa and T. J. Williams, of the
Western Electric Company, visited sev-
eral points along the Toledo-South Bend
cable, to inspect the equipment that is
used on this installation.

Joun Ma terr inspected the Fourth
Transcontinental Line between Albu-
querque and Whitewater, California, to
select test sites for Type-J carrier cross-
talk tests. Mr. Mallett and Joseph L.
Lindner are now making performance
tests of these Type-J transposed open-
wire circuits between Albuquerque, New
Mexico, and Holbrook, Arizona.

In CONNECTION with vacuum tube
trials, A. A. Heberlein visited the Long
Lines offices at Cincinnati, Atlanta,
Charlotte, and Washington during March,

H. E. Curtis and A. W. Lesert re.
turned recently from Albuquerque, New
Mexico, where they conducted trans-
mission tests on a new disc-insulated
spiral-four toll entrance cable.

W. H. Tipp is conducting tests between
Jacksonville and West Palm Beach in
connection with the trial installation of
the Type-J system.

DurING THE first quarter of 1938, the
following Laboratories employees have
been enrolled as members of the Tele-
phone Pioneers of America:

C. J. Beck E. L. Nelson
H. B. Brown Michael O’Connell
Miss G. R. Callender L. D. Plotner
L. P. Collins Erich von Nostitz
A. W. Horne Miss M. G. Reilly
J. B. Kelly E. C. Wente

H. W. Evans recently made transmis-
sion tests on a sample length of a new
type of tree wire at Chester.

L. R. Monrrorr inspected Type-G
carrier telephone systems in the vicinity
of Bluefield, West Virginia, and Cum-
berland, Maryland.

Recentiy, A. G. Chapman, W. E.
Mougey, J. F. Wentz and E. I. Green
visited Point Breeze to inspect and dis-
cuss the manufacture of disc-insulated
spiral-four cable and other types of cable.

H. A. ETHERIDGE made a trip to Dur-
ham to investigate return loss conditions
on the “B” cable between Petersburg
and Greensboro.

O. D. Grismore and P. A. JEANNE
visited Knoxville recently in connection
with an oscillograph installation on a
distribution circuit of the Tennessee
Public Service Company. This installa-
tion was made as part of the study of joint
use of power and telephone facilities at
the higher distribution voltages by the
Joint Subcommittee on Development and
Research of the Edison Electric Institute
and the Bell Telephone System.

it became apparent that im-
portant advantages would result if the
same channel terminal equipment
could be used for all systems. Such a
generally applicable terminal not only
would reduce the total amount of
development required, and the manu-
facturing preparation that would later
be needed, but would permit a larger
quantity production of the individual
apparatus units with its consequent
economies, and would give greater
facility of interconnection of different
types of systems. To obtain such a
common channel terminal arrange-

ment, numerous frequency alloca-
tions and groupings of channels were
studied, and a scheme was finally
worked out which meets the require-
ments of each system. Both the type
J, Or open-wire system, and the type
K, or cable system, provide twelve
carrier channels; and the coaxial
cable system, which will provide
several hundred channels, uses a
basic twelve-channel group, which is
raised by other modulations into suc-
cessive positions in the frequency
spectrum. The twelve-channel group
is thus common to all these systems,
each having different types of group
modulators, demodulators, and filters

with the appropriate carrier
supply to adapt the set of
twelve channels to any of the
three systems.

The channel terminal equip-
ment is practically identical
for all systems, and naturally
divides itself into two sections:
modulating and selecting cir-
cuits, and the carrier supply
circuits. The carrier supply
equipment provides twelve
carrier frequencies spaced 4000
cycles apart and extending
from 64 to 108 kilocycles,
inclusive, with certain other
frequencies for special pur-
poses. Sufficient capacity is
provided to supply a number
of groups from the same
carrier source. Ten groups, or
a total of 120 channels, may
be supplied from a single
carrier unit, and more could
be supplied if, at any time, it
should prove desirable.

Fig. 2—Installation of channel equipment located at
32 Sixth Avenue, New York

channel terminal. Each unit occupies
1214 inches on a standard relay rack
bay, and eighteen of them mounted
on two bays are shown in Figure 2 as

MODULATOR
2 MODU-| TO TRANS-
= LATOR| MITTING
3 BAND | GROUP
red FILTER| CIRCUITS
TO CARRIER

2 MODULATING
UNITS SUPPLY
x
\
= oc
pemoou-| DEMODU-| TO
° LATOR LATOR | RECEIVING
F AMPLI- BAND | _GROUP

Fig. 1—Schematic diagram of the channel terminal
equipment for a single two-way channel

set up for the experimental
coaxial demonstration at 32
Sixth Avenue. The bay at the
left of the channel terminal
units includes the carrier sup-
ply equipment.

The modulating and select-
ing equipment for a single two-
way telephone circuit is shown
in schematic form in Figure I.
Inthetransmitting side, above,
there are, from left to right,
a repeating coil, a copper-
oxide modulator, a resistance
network, and a_ band-pass
filter. On the receiving side

there are, from right to left, a band-
pass filter, a resistance network, a
copper-oxide demodulator, and the
demodulator amplifier. Equipment
for two of such circuits comprises the
unit panel shown in Figure 2. A small
jack strip, which projects through the
front cover as shown in the photo-
graph at the head of this article,
provides a simple means of reading
values of the currents and voltages of
the amplifier tubes.

One of the outstanding features of
this equipment is its simplicity and
small size. The modulator and de-
modulator each consist of four copper-
oxide discs only three-sixteenths inch
in diameter and connected in a
“bridge” network. This network may
be considered as a variable resistance
shunt across the signal circuit, which
changes from a low to a high re-
sistance under control of the carrier
frequency. Because of the balanced
form of this circuit, practically none
of the carrier appears in the output
side, which in the transmitting channel
includes the two sidebands only. For
the same reason none of the voice or
sideband frequencies can get into the
carrier supply to interfere with other
channels. The resistance network be-
tween modulator and filter serves to
give a constant impedance termina-
tion for the filter instead of the
varying impedance termination that
would be offered by the copper-oxide
network.

This balanced condition of the
modulator and demodulator is very
important, and is secured by careful
selection of the discs during manu-
facture. There are in reality two
types of balance to be considered. In
the first place, the four discs compris-
ing a single modulator are selected to
have the same resistance within very
close limits. This insures the balance

that prevents carrier getting into the
output circuits, or voice or sideband
into the carrier circuits. Besides this
form of balance, it is desirable that
the output of all channels be approxi-
mately at the same level. Since the
transmission loss through the modu-
lator is an important factor in de-
termining this level, the discs for the
modulator are further selected to
have substantially the same trans-
mission loss.

The filters are of the recently de-
veloped quartz-crystal type, and pro-
vide a pass-band about 3000 cycles
wide and with very steep sides, as
shown in Figure 3. This type of filter
is very economical of space; each
filter requiring only one side of a 3%4-

Fig. 3—Frequency characteristics of the
crystal filters for the channels

inch panel. Two of the four filters
required for the two channels are
mounted at the top and bottom of the
panel in the photograph at the head
of this article; the other two occupy
corresponding positions on the rear of
the panel. The twelve transmitting
filters of a group are multipled to-
gether at their output terminals,
together with a compensating net-
work to improve the impedance
termination. A similar network is
connected to the multipled terminals

of the twelve receiving filters. These
compensating networks are housed on
the narrow panels between the third
and fourth panels from the bottom
as seen in Figure 2.

To take care of losses that may be
interposed between the two-wire cir-
cuit at the toll board and the four-
wire circuit at the channel terminal,
it is necessary to have the level at the
receiving circuit higher than that at
the sending circuit. Since the same
carrier supply is employed for both
modulator and demodulator, however,
the output of the demodulator is at
a lower level than that of the input
circuits because of the losses in the
demodulator. An amplifier is there-
fore provided to raise the level of the
demodulated signal to the desired
level. Gain adjustment of the ampli-
fier is provided by a small potenti-
ometer. For all the channels, both
transmitting and receiving, the voice
circuits are terminated in jacks all
grouped at one location; and the gain
potentiometers of the demodulator
amplifiers are mounted above their
corresponding jacks. This makes a
very convenient arrangement for
maintenance, since the level of all
channels at the output of the carrier
equipment may be measured and ad-
justed in rapid succession at the
same place on the panel equipment.

Carrier current for the twelve
channels is provided by a harmonic.
producing circuit supplied from 4
40o0o-cycle source. The circuit ar.
rangement is shown in Figure 4. The
precision required of the basic supply
frequency depends on the highest
carrier frequency to be supplied. For
the type J and kK systems, with an
upper frequency of about 60 and 140
kilocycles respectively, the precision
does not need to be so great as for the
coaxial system where the upper
carrier is a million or more cycles. A
400o0-cycle tuning fork is adequate for
the low-frequency systems, while for
the coaxial system, the high-precision
standard frequency of 4000 cycles now
supplied from New York may be
employed. The alternative connec-
tions are indicated on the diagram.
The fork is actually employed in both
cases, but for the low-frequency
systems it is used to control the
oscillator frequency, while for the
high-frequency system it is used only
as a 4-kc band-pass filter.

The output of the oscillator passes
to a single-tube amplifying stage, and
thence to a push-pull stage and an
output transformer. This transformer
is tuned to 4000 cycles on its output
side, and it feeds the harmonic-pro-
ducing circuit through another tuning
stage to insure a pure 4000-cycle wave.

Fig. 4—Simplified schematic of the carrier-supply circuit for the channel terminals of
broad-band carrier systems

The harmonic-producing circuit con-
sists of a shunt coil, two series
condensers, and a copper-oxide bridge
circuit. At the beginning of the
current wave the coil is of high in-
ductance, and the condensers charge,
but as the current increases, the core
of the coil becomes saturated, and as
a result the inductance decreases so
that the coil approxi- |

odd harmonics of 8000 cycles are even
harmonics of 4000 cycles, this circuit
produces the even harmonics of the
4000-cycle fundamental, which are
selected by a second group of filters.
The requirements placed on these
filters are made easier because of the
balanced arrangement of the copper-
oxide discs, which tends to prevent

PATCHING
This transition from
high inductance to MoDU- TO TRANS-
lowoccurs rapidly with circuits | | BAND
the result that the HT
condenser discharges |
through the coil in a | conapen-
rush of current. The NETWORK
result is ” sharply TO OTHER MODULATORS
peaked current wave, AND, DEMODUL ATORS
decomposable into a ‘eee
4ooo-cycle fundamen-
tal and its odd har- “FILTERS.
monics. The odd har- t

monics desired are se-
lected by a group of
filters as indicated on
the diagram.

Even harmonics are
produced by the
copper-oxide _ bridge,
which, acting as a full-
wave rectifier, causes a
series of current peaks
in its output branch.
There is a peak for

same sign because of
the rectifying action
of the bridge. A cur-
rent of this kind is

COMPEN-
SATING
NETWORK
DEMODU- TO
DEMODU- LATOR RECEIVING
LATOR BAND GROUP.
FIER FILTER EQUIPMENT

decomposable into a
fundamental of 8000
cycles and all har-
monics, both even and
odd. Since even and

Fig. s—Simplified schematic of channel terminal unit for
broad-band systems

The filters used to select the carrier
frequencies are also of the crystal
type, but contain only one section.
Beyond them, a set of resistances is
provided through which connection
is made to the various circuits as indi-
cated in Figure 5. These resistances
protect the common carrier supplies
against possible short circuit in any
lead. As mentioned above certain
other harmonics may be employed
also, such as the 24-ke current that

was used as the fundamental fre.
quency for the production of carrier
frequencies in the million-cycle coaxial
system trial, and the 60-ke current used
as a pilot frequency.

A photograph of a carrier-supply
bay, with the protective resistances
above, is shown in Figure 6 as set up
in the Laboratories for the coaxial
tests. Two harmonic-producing cir-
cuits, one a_ regular and one an
emergency, are normally provided to
safeguard the carrier supply. Means
are provided for automatically trans-
ferring from the regular to the emer-
gency in case of failure, and of manu-
ally transferring in either direction
with very slight interruption to any
of the circuits involved.

Of the many interesting charac-
teristics of this channel-terminal
equipment, one of the most impressive
is that of its compactness. Due to the
use of crystal filters, of small size coils
with improved core material, and of
the copper-oxide modulator and de-
modulator units, the complete channel
modulating and selecting equipment
for three groups, each of twelve
two-way telephone circuit capacity,
can be mounted on two relay rack
bays, as shown in Figure 2. All the
wiring between units is arranged to
run up both sides of the relay racks
in metal compartments which provide
shielding. This permits the use of
more unshielded wire than would be
possible otherwise.

The first use of this equipment was
with the experimental coaxial cable
system between New York and Phila-
delphia, which was demonstrated to
the press in November, 1936.

illuminated, the current
through it increases very rapidly at
first but attains its maximum value
after only about a thousandth of a
second. Likewise when the light is
abruptly extinguished some current
flows through the darkened cell for
about the same brief interval. This
delay has been ascribed to the time
required for the gas ions which are
generated in the tube to move to the
cathode. The failure of the output
to follow the light variation exactly
can be explained readily on this
assumption at frequencies of about
10,000 cycles; but considerable doubt
has been expressed that the lag at
lower frequencies could be due to the
same cause.

The current in a gaseous cell in-
volves not only the electrons which
the light liberates at the cathode but
also all the other electrons and ions

which these photoelectrons produce
in collision with gas molecules. Thus
for each primary photoelectron liber-
ated at the cathode, a number of
electrons arrive at the anode. The
positive ion formed at each ionizing
collision travels in the opposite direc-
tion toward the cathode, but moves
much more slowly through the gas
because of its larger size and mass.
In studies in gas-filled cells com-
mercial designs are not particularly
suitable, mainly because in them the
electric field is not uniform and hence
some ions take longer to reach the
cathode than others. The design
shown in Figure 3 overcomes this de-
fect, for its anode is small and all parts
of the cathode are approximately
equidistant from it. As a consequence
of this geometrical form, the electric
field is distributed uniformly through-
out the entire tube as shown in
Figure 2, and the transit time of all
the ions is approximately the same.

Fig. 1\—The magnitude of the alternating current output of the other pulse of current
cell varies with the frequency of the incident light because the to flow. The second
components of the current carried by the electrons and the pulse flows after a time

If the photosensitive cathode 1s
illuminated by light whose intensity
varies sinusoidally, the emission of the
primary photoelectrons and the con-
sequent production of positive ions
will also vary sinusoidally.
Waves of ions will travel from
the region of the anode to the
cathode, slowing up as they
approach the cathode like
ocean waves rolling in on a
sandy beach. Now it so hap-
pens that the current in the
external connecting circuit due
to the transit of a charged
particle across the cell flows
during the whole time the
particle is in motion, ceasing
when it delivers up its charge
on reaching an electrode. Fur-
thermore, most of this current
flows while the particle (ion or
electron) is in the vicinity of
the anode and is moving
swiftly. The electrons get

proximately equal to
the transit time of the ions and suc-
ceeding pulses of decreasing ampli-
tude follow at intervals equal to this.
Thus if the release of photoelectrons
varies sinusoidally, a series of sinu-

across the cell very quickly Fig. 2—The symmetrical construction of the photo-
and the ions get out of the electric cell gives a uniform distribution of the

soidally varying currents will flow in
the external circuit. These will all be
in phase when the transit time of the
ions is equal to the period of the cur-
rent variations, for then the crests of
the waves fall together and the total
alternating current output is a maxi-
mum. Conversely, when the transit
time of the photoelectrons is equal to
half a period, the total current falls
to a minimum value.

By measuring the magnitude of the
output of the cell for different fre-
quencies of light variation and plot-
ting the data, Figure 1, the first
minimum and subsequent maximum
are found to occur where expected on
the assumption that the effect is due
to the relatively slow velocity of the
gas ions. Other maxima and minima
might be anticipated at higher fre-
quencies, for which the times of flight
would be multiples of the periods and
half periods. They are not observable,
however, because the ions move about
in a very irregular manner with the
result that at the higher frequencies
individual ions wander from their
positions in the wave, the crests and
troughs merge, and the waves lose
their identity.

It is possible to calculate, from the
speed at which argon ions drift, what
the average velocity of the ions at
different distances along the radius
should be; and to deduce therefrom
the total time taken by an ion in going
from the vicinity of the anode to the
cathode. As stated in the preceding
paragraph the value thus determined
is the same as the period of the light
frequency at the first maximum, or
half the period at the first minimum.
This confirms the theory given above
and thus establishes the nature of the
time lag in this cell.

_ In commercial cells large variations
in the time delay are to be expected be-

cause of their non-uniform fields, men-
tioned earlier in the article. Thus the
fact that maxima and minima have
not been observed in curves taken

small and all parts of the cathode are ap-
proximately equidistant from it

with commercial cells of usual design
is reconcilable with the theory that
has been developed. But the lag in
response is there just the same.

For the voice-frequency band used
in sound pictures the distortion caused
by the lag in the photoelectric cell is
not serious. There is some phase dis-
tortion but fortunately the ear can-
not detect it. The most obvious
practical consequence of the lag is a
small loss in efficiency which, now ex-
plained, could be reduced if necessary
by modifying the cells. Where fre-
quencies much above 10,000 cycles
are involved the lag becomes large
and has to be taken into consideration
in the design of cells.

VERY year approximately a
half million new poles of south-
ern yellow pine are used in the
Bell System either for replacements or
for new lines. If these poles were left
unprotected from the action of lower
forms of plant and animal life—fungi
and termites—the economic loss would
be serious. Their life can be greatly
extended, however, by treating the
poles with a suitable preservative.
There are many preservatives on the
market, and new ones are being
offered from time to time to pole
manufacturers, so that techniques for
evaluating them must be available.
Outdoor exposure tests of wood
preservatives are usually slow and
expensive and it is necessary to sup-
plement such studies with quick and
inexpensive laboratory methods.
Recent experimentation has evolved
an improved laboratory test which is
simple and adaptable and yet gives

reproducible results which are con-
sistent with field trials of the same
material. Briefly, it involves the im-
pregnation of small pieces of blocks of
wood with the preservative to be
tested, and their exposure in separate
glass jars to the action of different
fungi. After several weeks, the pre-
servative’s effectiveness is rated by
the amount and density of the fun-
gus growth, the decrease of weight
of the samples, and their loss of
mechanical strength.

The blocks used in testing preserva-
tives are from the sap-wood of south-
ern yellow pine, of uniform density
and rate of growth, cut usually into
cubes, two centimeters on a side.
Since the moisture content of the wood
varies over a wide range, depending
on the relative atmospheric humidity
to which it has been exposed, the
blocks have to be brought to a definite
humidity content before each weigh-

ing at the various stages of the test.
For this purpose there is used an
ordinary bacteriological incubator,
Figure 3, fitted with slow-moving fans
and pans of saturated solution of
sodium chloride. It maintains a rela-
tive humidity of 76 per cent at 30 de-
grees Centigrade which gives the
blocks a moisture content of about
14.1 per cent when compared with the
oven-dry state.

The procedure usually followed in
studying a new preservative is to in-
ject blocks, conditioned as above,
with a solution of the preservative.
The pieces are weighed immediately
after impregnation to determine the
amount of preservative injected, and
after evaporation of the solvent they
are reweighed under the standard con-
ditions, referred to above, to serve as a
further check on their actual content
of preservative. This is of extreme
importance in working with volatile
preservatives.

Each impregnated block is sup-
ported by a thin slab of untreated
wood on the top of a small wide-
mouthed bottle which contains water.
The slab also acts as a secondary food

source for the growth of the fungus.
Wooden surgical applicators, cut in
half, are used to anchor the wood and
to serve as wicks for conducting water
through the test pieces, Figure 2. For
protection the small bottles are
wrapped in cotton and placed in
larger bottles with screw caps. The
complete set-up is sterilized for fifteen
minutes at fifteen pounds pressure and
then cooled to room temperature.
Then a small section, cut from a pure
culture of a wood-destroying fungus,
Figure 1, is placed on the untreated
slab of wood opposite the test speci-
men. The bottles are kept in an incu-
bation room, and observed every four
weeks for the extent of the fungus
growth. After an exposure to the
fungus of about twenty-four weeks
the blocks are again brought to equi-
librium in the humidity chamber and
weighed. The loss in weight so deter-
mined serves as one of the indications
of the efficacy of the preservative that
has been employed.

At the present time some six hun-
dred species of fungi are known to at-
tack wood, and since all cannot be
used in each test for preservatives

Fig. 1—Component parts of the apparatus used for the wood-block assay of preservative
materials. These tests are carried on at the Summit Laboratory

care must be exercised to select repre-
sentative fungi. Previous experience
has shown that in addition to extreme
variability in virulence to different
species of wood the fungi display
marked idiosyncrasies towards var-
ious preservatives. On the basis of

Fig. 2—Test apparatus assembled and the
wood inoculated with a small piece of
fungus culture

these two factors, many of the fungi
chosen have been found in southern
pine telephone poles which had failed
in service.

One of these, the fungus Lentinus
lepideus, has been repeatedly found on
test specimens in the Laboratories’
test “garden” at Gulfport, Mississippi,
and on poles in widely separated areas
in the United States. It possesses a
marked resistance to certain organic

preservatives, and is mentioned by
several investigators as being of wide-
spread economic importance in the
decay of building and structural tim-
bers. Another fungus, Lenzites trabea,
also exerts a strong resistance to or-
ganic preservatives but since the type
of decay produced is unique in that it
takes place primarily at the surface,
this fungus is also included in most
tests. The resistance of Fomes roseys
varies greatly, but this flat-growing
and innocent-looking fungus masks an
occasional virulence which makes it
an essential member of every test.
Another organism included in all
tests has not been identified as yet,
despite attempts by several mycolog-
ical authorities. It masquerades un-
der the designation U-1o. Isolated a
few years ago from a decayed pine
telephone pole, it is especially valu-
able when a quick indication of the

value of a new pre-
servative is needed, as
it is capable of pro-
ducing a very appreci-
able weight loss in
about three months.
When preservatives of
the inorganic type are
under consideration
common dry-rot fungi
are used because of
their specific resist-
ance to this particular
class of compounds.
From time to time
other wood-destroying
organisms have sup-
plemented those in
regular use.

To aid in interpret-
ing the results of tests
on the efficacy of a
preservative, un-
treated controls are
also exposed to the
different test fungi;
and treated controls
are put through the
entire test cycle with-
out inoculation. These
controls are valuable
indicators of the na-
ture of the preservative from the
standpoint of solubility, volatility and
chemical stability; and particularly of
the strength that is lost by the ex-
posed test blocks.

Three criteria are available as
measures of the value of a preserva-
tive. First there is the growth rating
which is made every four weeks with
reference to the untreated norm. This
rating is designated by a pair of num-
bers, the first of which indicates the
extent of the test block covered with
the fungus, and the second the in-
tensity and vigor of the growth.
Based on 4 as the maximum, 2-4

Fig. 4—Incubation room where the bottles are stored during
the twenty-four-week test period at a constant temperature of
twenty-six degrees Centigrade

would mean that the block was partly
covered with normal growth and 4-2
wholly covered with sparse growth.
A second measure is the weight loss
computed from the equilibrium
weights before and after exposure to
the fungus, with corrections for leach-
ing, volatility or other causes as mani-
fested in the uninoculated but treated
test-blocks. The third basis for judg-
ing the merits of the preservative is
the dissection or strength rating de-
termined by breaking the blocks in
small pieces and comparing them with
the uninoculated controls that had
been treated in a similar manner.

Fig. s—Test of a preservative with the wood-block method showing the effect of increasing
concentration on the growth of the test fungus Lenzites trabea. Growth ratings of these
typical test blocks from left to right are 4-4, 4-3, 3-2, 0-0

This modification of previous wood-
block test methods is no doubt cap-
able of still further development, but
several years’ experience with it has
provided case histories for many pre-
servatives which show a gratifying
correlation between results of this ac-

celerated and inexpensive laboratory
test and of the slower and more ex-
pensive field trials. The severity of the
method may in some cases be criti-
cized but this very severity is essential
in the elimination of mediocre ma-
terials unworthy of further study.

F. A. Zupa came to the Laboratories in
1918, and engaged first in testing and
development work on materials and tele-
phone apparatus in the Physical Labora-
tory until 1924. This included two years
on photomicrographic and microscopic
analysis of materials. Since 1924, he has
been engaged in the design and develop-
ment of telephone relays; recently he has
devoted most of his time to the U and Y
types. He obtained his technical educa-
tion at the College of the City of New
York and Cooper Union, receiving the
degree of B.S. in Electrical Engineering
from the latter school in 1922.

H. N. Wacar received the S.B. de-
gree in physics from Harvard University
in 1926, and shortly afterward joined
these Laboratories. As a member of the
Apparatus Development Department
since that time he has participated in de-
velopment work on practically all classes
of telephone relays. During this period he
also continued his studies at Columbia
University, and received the M.A. de-
gree in 1931. In the course of his studies
of dynamic problems relating to relays, he
developed the first oscillographic system

for obtaining simultaneous records of
mechanical and electrical vibrations in
electromechanical devices, and has made
several contributions to this rapidly
growing technique. He is at present asso-
ciated with development work on relays
and magnet coils intended for manufac-
ture on a large scale.

J. R. Pierce received his B.S. degree
in 1933 from the California Institute of
Technology, and from the same insti-
tution received an M.S. in 1934 and a
Ph.D. in Electrical Engineering in 1936.
He then joined the Technical Staff of Bell
Telephone Laboratories where he has
been engaged in the development of
various types of vacuum tubes.

Joun Leurrirz received the degree of
B.S. in Chemistry from Bowdoin College
in 1929 and an A.M. in Botany from
Columbia University in 1934. From 1921
to 1925 he was with the U. S. Navy
Medical Corps. He joined the Bell Tele-
phone Laboratories in 1929 and has been
engaged in research along biological lines
primarily relating to wood preservation.

immediately joined the Engineering De-
partment of the Western Electric Com-
pany. He has been engaged since that
time in development work on carrier tele-
phone systems. For the last several years
he has been in charge of a group which
has been concerned with the terminals of
broad-band carrier systems, principally
those for application to voice-frequency
and coaxial cables.

A. M. Skettetr joined the Labora-
tories in 1929 after spending several
years teaching, finally as instructor and
assistant professor of physics at the Uni-

versity of Florida. He also worked one
summer as physicist with the Westing-
house Company and served as chief
engineer of Station WRUF. Long-wave
antenna design and general problems of
radio transmission occupied Dr. Skellett’s
time when he first came to the Labora-
tories. Since 1934 he has been with the
physical research group studying the ap-
plication of atomic and electronic devices
to telephony. Dr. Skellett received the
A.B. and M.S. degrees from Washington
University in 1924 and 1927, respectively,
and the Ph.D. from Princeton in 1933.

Bell Laboratories Record 1938-05: Vol 16 Iss 9