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Types of Central-Office Protectors. A form of combined heat coil and
air-gap arrester, widely used by Bell companies for central-office
protection, is shown in Fig. 226. The two inner springs form the
terminals for the two limbs of the metallic-circuit line, while the
two outside springs are terminals for the continuation of the line
leading to the switchboard. The heat coils, one on each side, are
supported between the inner and outer springs. High-tension currents
jump to ground through the air-gap arrester, while sneak currents
permit the pin of the heat coil to slide within the sleeve, thus
grounding the outside line and the line to the switchboard.
[Illustration: Fig. 226. Sneak-Current and Air-Gap Arrester]
_Self-Soldering Heat Coils._ Another form designed by Kaisling and
manufactured by the American Electric Fuse Company is shown in Fig.
227. In this the pin in the heat coil projects unequally from the ends
of the coil, and under the action of a sneak current the melting of
the solder which holds it allows the outer spring to push the pin
through the coil until it presses the line spring against the ground
plate and at the same time opens the path to the switchboard. When the
heat-coil pin assumes this new position it cools off, due to the
cessation of the current, and _resolders_ itself, and need only be
turned end for end by the attendant to be reset. Many are the
variations that have been made on this self-soldering idea, and there
has been much controversy as to its desirability. It is certainly a
feature of convenience.
[Illustration: Fig. 227. Self-Soldering Heat-Coil Arrester]
Instead of using a wire-wound resistance element in heat-coil
construction some manufacturers employ a mass of high-resistance
material, interposed in the path of the current. The Kellogg Company
has long employed for its sneak-current arrester a short graphite rod,
which forms the resistance element. The ends of this rod are
electroplated with copper to which the brass terminal heads are
soldered. These heads afford means for making the connection with the
proper retaining springs.
[Illustration: Fig. 228. Cook Arrester]
Another central-office protector, which uses a mass of special metal
composition for its heat producing element is that designed by Frank B.
Cook and shown in Fig. 228. In this the carbon blocks are cylindrical
in form and specially treated to make them “self-cleaning.” Instead of
employing a self-soldering feature in the sneak-current arrester of
this device, Cook provides for electrically resoldering them after
operation, a clip being designed for holding the elements in proper
position and passing a battery current through them to remelt the
solder.
In small magneto exchanges it is not uncommon to employ combined fuse
and air-gap arresters for central-office line protection, the fuses
being of the mica-mounted type already referred to. A group of such
arresters, as manufactured by the Dean Electric Company, is shown in
Fig. 229.
[Illustration: Fig. 229. Mica Fuse and Air-Gap Arresters]
Types of Subscribers Station Protectors. Figs. 230 and 231 show types
of subscribers station protectors adapted to the requirements of
central-battery and magneto systems. These, as has been said, should be
mounted at or near the point of entrance of the subscribers line into
the premises, if the line is exposed outside of the premises. It is
possible to arrange the fuses so that they will be safe and suitable
for their purposes if they are mounted out-of-doors near the point of
entrance to the premises. The sneak-current arrester, if one exists,
and the carbon arrester also, must be mounted inside of the premises or
in a protecting case, if outside, on account of the necessity of
shielding both of these devices from the weather. Speaking generally,
the wider practice is to put all the elements of the subscribers
station protector inside of the house. It is nearer to the ideal
arrangement of conditions if the protector be placed immediately at the
point of entrance of the outside wires into the building.
[Illustration: Fig. 230. Western Electric Station Arrester]
[Illustration: Fig. 231. Cook Arrester for Magneto Stations]
_Ribbon Fuses_. A point of interest with relation to tubular fuses is
that in some of the best types of such fuses, the resistance material
is not in the form of a round wire but in the form of a flat ribbon.
This arrangement disposes the necessary amount of fusible metal in a
form to give the greatest amount of surface while a round wire offers
the least surface for a given weight of metal–a circle encloses its
area with less periphery than any other figure. The reason for giving
the fuse the largest possible surface area is to decrease the
likelihood of the fuse being ruptured by lightning. The fact that such
fuses do withstand lightning discharges much more thoroughly than
round fuses of the same rating is an interesting proof of the
oscillating nature of lightning discharges, for the density of the
current of those discharges is greater on and near the surface of the
conductor than within the metal and, therefore, flattening the fuse
increases its carrying capacity for high-frequency currents, without
appreciably changing its carrying capacity for direct currents. The
reason its capacity for direct currents is increased at all by
flattening it, is that the surface for the radiation of heat is
increased. However, when enclosed in a tube, radiation of heat is
limited, so that for direct currents the carrying capacity of fuses
varies closely with the area of cross-section.

In order that the armature and cores may be normally polarized, a
permanent magnet _6_ is secured to the center of the yoke piece _1_.
This bends around back of the electromagnets and comes into close
proximity to the armature _5_. By this means one end of each of the
electromagnet cores is given one polarity–say north–while the
armature is given the other polarity–say south. The two coils of the
electromagnet are connected together in series in such a way that
current in a given direction will act to produce a north pole in one
of the free poles and a south pole in the other. If it be assumed that
the permanent magnet maintains the armature normally of south polarity
and that the current through the coils is of such direction as to make
the left-hand core north and the right-hand core south, then it is
evident that the left-hand end of the armature will be attracted and
the right-hand end repelled. This will throw the tapper rod to the
right and sound the right-hand bell. A reversal in current will
obviously produce the opposite effect and cause the tapper to strike
the left-hand bell.
An important feature in polarized bells is the adjustment between the
armature and the pole pieces. This is secured in the Western Electric
bell by means of the nuts _7_, by which the yoke _4_ is secured to the
standards _3_. By moving these nuts up or down on the standards the
armature may be brought closer _to_ or farther _from_ the poles, and
the device affords ready means for clamping the parts into any
position to which they may have been adjusted.
[Illustration: Fig. 79. Polarized Bell]
_Kellogg Ringer._ Another typical ringer is that of the Kellogg
Switchboard and Supply Company, shown in Fig. 80. This differs from
that of the Western Electric Company mainly in the details by which
the armature adjustment is obtained. The armature supporting yoke _1_
is attached directly to the cores of the magnets, no supporting side
rods being employed. Instead of providing means whereby the armature
may be adjusted toward or from the poles, the reverse practice is
employed, that is, of making the poles themselves extensible. This is
done by means of the iron screws _2_ which form extensions of the
cores and which may be made to approach or recede from the armature by
turning them in such direction as to screw them in or out of the core
ends.
[Illustration: Fig. 80. Polarized Bell]
[Illustration: Fig. 81. Biased Bell]
_Biased Bell._ The pulsating-current generator has already been
discussed and its principle of operation pointed out in connection
with Fig. 77. The companion piece to this generator is the so-called
biased ringer. This is really nothing but a common alternating-current
polarized ringer with a light spring so arranged as to hold the
armature normally in one of its extreme positions so that the tapper
will rest against one of the gongs. Such a ringer is shown in Fig. 81
and needs no further explanation. It is obvious that if a current
flows in the coils of such a ringer in a direction tending to move the
tapper toward the left, then no sound will result because the tapper
is already moved as far as it can be in that direction. If, however,
currents in the opposite direction are caused to flow through the
windings, then the electromagnetic attraction on the armature will
overcome the pull of the spring and the tapper will move over and
strike the right-hand gong. A cessation of the current will allow the
spring to exert itself and throw the tapper back into engagement with
the left-hand gong. A series of such pulsations in the proper
direction will, therefore, cause the tapper to play between the two
gongs and ring the bell as usual. A series of currents in a wrong
direction will, however, produce no effect.

Station A the upper conductor, Fig. 170, is connected to binding post
_1_ and the lower conductor to binding post _2_, while at Station B
the upper conductor is connected to binding post _2_ and the lower
conductor to binding post _1_. The permanent wiring of this telephone
set is the same as that frequently used for a set connected to a line
having only one station, the proper ringing circuit being made by the
method of connecting up the binding posts. For example, if this
telephone set were to be used on a single station line, the binding
posts _1_ and _2_ would be connected to the two conductors of the line
as before, while binding post _3_ would be connected to post _1_
instead of being grounded.
[Illustration: Fig. 175. Circuit of Two-Party Station]
_Circuits of Four-Party-Line Telephones._ The wiring of the telephone
set used with the system illustrated in Fig. 172 is shown in detail in
Fig. 176. The wiring of this set is arranged for local battery or
magneto working, as this method of selective ringing is more frequently
employed with magneto systems, on account of the objectionable features
which arise when applied to common-battery systems. In this figure the
line conductors are connected to binding posts _1_ and _2_, and a
ground connection is made to binding post _3_. In order that all sets
may be wired alike and yet permit the instrument to be connected for
any one of the various stations, the bell is not permanently wired to
any portion of the circuit but has flexible connections which will
allow of the set being properly connected for any desired station. The
terminals of the bell are connected to binding posts _9_ and _10_, to
which are connected flexible conductors terminating in terminals _7_
and _8_. These terminals may be connected to the binding posts _4_,
_5_, and _6_ in the proper manner to connect the set as an A, B, C, or
D station, as required. For example, in connecting the set for Station
A, Fig. 172, terminal _7_ is connected to binding post _6_ and _8_ to
_5_. For connecting the set for Station B terminal _7_ is connected to
binding post _5_ and _8_ to _6_. For connecting the set for Station C
terminal _7_ is connected to binding post _6_ and _8_ to _4_. For
connecting the set for Station D terminal _7_ is connected to binding
post _4_ and _8_ to _6_.
[Illustration: Fig. 176. Circuit of Four-Party Station without Relay]
[Illustration: Fig. 177. Circuit of Four-Party Station with Relay]
The detailed wiring of the telephone set employed in connection with
the system illustrated in Fig. 173 is shown in Fig. 177. The wiring of
this set is arranged for a common-battery system, inasmuch as this
arrangement of signaling circuit is more especially adapted for
common-battery working. However, this arrangement is frequently
adapted to magneto systems as even with magneto systems a permanent
ground connection at a subscribers station is objectionable inasmuch
as it increases the difficulty of determining the existence or
location of an accidental ground on one of the line conductors. The
wiring of this set is also arranged so that one standard type of
wiring may be employed and yet allow any telephone set to be connected
as an A, B, C, or D station.
Harmonic Method. _Principles._ To best understand the principle of
operation of the harmonic party-line signaling systems, it is to be
remembered that a flexible reed, mounted rigidly at one end and having
its other end free to vibrate, will, like a violin string, have a
certain natural period of vibration; that is, if it be started in
vibration, as by snapping it with the fingers, it will take up a
certain rate of vibration which will continue at a uniform rate until
the vibration ceases altogether. Such a reed will be most easily
thrown into vibration by a series of impulses having a frequency
corresponding exactly to the natural rate of vibration of the reed
itself; it may be thrown into vibration by very slight impulses if
they occur at exactly the proper times.
It is familiar to all that a person pushing another in a swing may
cause a considerable amplitude of vibration with the exertion of but a
small amount of force, if he will so time his pushes as to conform
exactly to the natural rate of vibration of the swing. It is of course
possible, however, to make the swing take up other rates of vibrations
by the application of sufficient force. As another example, consider a
clock pendulum beating seconds. By gentle blows furnished by the
escapement at exactly the proper times, the heavy pendulum is kept in
motion. However, if a person grasps the pendulum weight and shakes it,
it may be made to vibrate at almost any desired rate, dependent on the
strength and agility of the individual.

By reactive interference is meant action whereby the transmitter
element, in emitting a wave, affects its own controlling receiver
element, thus setting up an action similar to that which occurs when
the receiver of a telephone is held close to its transmitter and
humming or singing ensues. No repeater is successful unless it is free
from this reactive interference.
[Illustration: Fig. 37. Mercury-Arc Telephone Relay]
Enough has been accomplished by practical tests of the Shreeve device
and others like it to show that the search for a method of relaying
telephone voice currents is not looking for a pot of gold at the end
of the rainbow. The most remarkable truth established by the success
of repeaters of the Shreeve type is that a device embodying so large
inertia of moving parts can succeed at all. If this mean anything, it
is that a device in which inertia is absolutely eliminated might do
very much better. Many of the methods already proposed by inventors
attack the problem in this way and one of the most recent and most
promising ways is that of Mr. J.B. Taylor, the circuit of whose
telephone-relay patent is shown in Fig. 37. In it, _1_ is an
electromagnet energized by voice currents; its varying field varies an
arc between the electrodes _2-2_ and _3_ in a vacuum tube. These
fluctuations are transformed into line currents by the coil _4_.
CHAPTER V
TRANSMITTERS
Variable Resistance. As already pointed out in Chapter II, the
variable resistance method of producing current waves, corresponding
to sound waves for telephonic transmission, is the one that lends
itself most readily to practical purposes. Practically all telephone
transmitters of today employ this variable-resistance principle. The
reason for the adoption of this method instead of the other possible
ones is that the devices acting on this principle are capable, with
great simplicity of construction, of producing much more powerful
results than the others. Their simplicity is such as to make them
capable of being manufactured at low cost and of being used
successfully by unskilled persons.

The relation between the windings of the induction coil in this
practice are such that the secondary winding contains many more turns
than the primary winding. Changes in the circuit of the primary
winding produce potentials in the secondary winding correspondingly
higher than the potentials producing them. These secondary potentials
depend upon the _ratio_ of turns in the two windings and therefore,
within close limits, may be chosen as wished. High potentials in the
secondary winding are admirably adapted to transmit currents in a
high-resistance line, for exactly the same reason that long-distance
power transmission meets with but one-quarter of one kind of loss when
the sending potential is doubled, one-hundredth of that loss when it
is raised tenfold, and similarly. The induction coil, therefore,
serves the double purpose of a step-up transformer to limit line
losses and a device for vastly increasing the range of change in the
transmitter circuit.
Fig. 13 is offered to remind the student of the action of an induction
coil or transformer in whose primary circuit a direct current is
increased and decreased. An increase of current in the local winding
produces an impulse of _opposite_ direction in the turns of the
secondary winding; a decrease of current in the local winding produces
an impulse of _the same_ direction in the turns of the secondary
winding. The key of Fig. 13 being closed, current flows upward in the
primary winding as drawn in the figure, inducing a downward impulse of
current in the secondary winding and its circuit as noted at the right
of the figure. On the key being opened, current ceases in the primary
circuit, inducing an upward impulse of current in the secondary
winding and circuit as shown. During other than instants of opening
and closing (changing) the local circuit, no current whatever flows in
the secondary circuit.
[Illustration: Fig. 13. Induction-Coil Action]
It is by these means that telephone transmitters draw direct current
from primary batteries and send high-potential alternating currents
over lines; the same process produces what in Therapeutics are called
“Faradic currents,” and enables also a simple vibrating contact-maker
to produce alternating currents for operating polarized ringers of
telephone sets.

Several important points must be borne in mind in the design of the
hook switch. The spring provided to lift the hook must be sufficiently
strong to accomplish this purpose and yet must not be strong enough to
prevent the weight of the receiver from moving the switch to its other
position. The movement of this spring must be somewhat limited in
order that it will not break when used a great many times, and also it
must be of such material and shape that it will not lose its
elasticity with use. The shape and material of the restoring spring
are, of course, determined to a considerable extent by the length of
the lever arm which acts on the spring, and on the space which is
available for the spring.
The various contacts by which the circuit changes are brought about
upon the movement of the hook-switch lever usually take the form of
springs of German silver or phosphor-bronze, hard rolled so as to have
the necessary resiliency, and these are usually tipped with platinum
at the points of contact so as to assure the necessary character of
surface at the points where the electric circuits are made or broken.
A slight sliding movement between each pair of contacts as they are
brought together is considered desirable, in that it tends to rub off
any dirt that may have accumulated, yet this sliding movement should
not be great, as the surfaces will then cut each other and, therefore,
reduce the life of the switch.
Contact Material. On account of the high cost of platinum, much
experimental work has been done to find a substitute metal suitable
for the contact points in hook switches and similar uses in the
manufacture of telephone apparatus. Platinum is unquestionably the
best known material, on account of its non-corrosive and
heat-resisting qualities. Hard silver is the next best and is found in
some first-class apparatus. The various cheap alloys intended as
substitutes for platinum or silver in contact points may be dismissed
as worthless, so far as the writers somewhat extensive investigations
have shown.
In the more recent forms of hook switches, the switch lever itself
does not form a part of the electrical circuit, but serves merely as
the means by which the springs that are concerned in the switching
functions are moved into their alternate cooperative relations. One
advantage in thus insulating the switch lever from the
current-carrying portions of the apparatus and circuits is that, since
it necessarily projects from the box or cabinet, it is thus liable to
come in contact with the person of the user. By insulating it, all
liability of the user receiving shocks by contact with it is
eliminated.
Wall Telephone Hooks. _Kellogg._ A typical form of hook switch, as
employed in the ordinary wall telephone sets, is shown in Fig. 83,
this being the standard hook of the Kellogg Switchboard and Supply
Company. In this the lever _1_ is pivoted at the point _3_ in a
bracket _5_ that forms the base of all the working parts and the means
of securing the entire hook switch to the box or framework of the
telephone. This switch lever is normally pressed upward by a spring
_2_, mounted on the bracket _5_, and engaging the under side of the
hook lever at the point _4_. Attached to the lever arm _1_ is an
insulated pin _6_. The contact springs by which the various electrical
circuits are made and broken are shown at _7_, _8_, _9_, _10_, and
_11_, these being mounted in one group with insulated bushings between
them; the entire group is secured by machine screws to a lug
projecting horizontally from the bracket _5_. The center spring _9_
is provided with a forked extension which embraces the pin _6_ on the
hook lever. It is obvious that an up-and-down motion of the hook lever
will move the long spring _9_ in such manner as to cause electrical
contact either between it and the two upper springs _7_ and _8_, or
between it and the two lower springs _10_ and _11_. The hook is shown
in its raised position, which is the position required for talking.

Electrolytic hazards depend not on the heating effects of currents but
on their chemical effects. The same natural law which enables primary
and secondary batteries to be useful provides a hazard which menaces
telephone-cable sheaths and other conductors. When a current leaves a
metal in contact with an electrolyte, the metal tends to dissolve into
the electrolyte. In the processes of electroplating and electrotyping,
current enters the bath at the anode, passes from the anode through
the solution to the cathode, removing metal from the former and
depositing it upon the latter. In a primary battery using zinc as the
positive element and the negative terminal, current is caused to pass,
within the cell, from the zinc to the negative element and zinc is
dissolved. Following the same law, any pipe buried in the earth may
serve to carry current from one region to another. As single-trolley
tractiosystems with positive trolley wires constantly are sending
large currents through the earth toward their power stations, such a
pipe may be of positive potential with relation to moist earth at some
point in its length. Current leaving it at such a point may cause its
metal to dissolve enough to destroy the usefulness of the pipe for its
intended purpose.
Lead-sheathed telephone cables in the earth are particularly exposed
to such damage by electrolysis. The reasons are that such cables often
are long, have a good conductor as the sheath-metal, and that metal
dissolves readily in the presence of most aqueous solutions when
electrolytic differences of potential exist. The length of the cables
enables them to connect between points of considerable difference of
potential. It is lack of this length which prevents electrolytic
damage to masses of structural metal in the earth.
Electrical power is supplied to single-trolley railroads principally
in the form of direct current. Usually all the trolley wires of a city
are so connected to the generating units as to be positive to the
rails. This causes current to flow from the cars toward the power
stations, the return path being made up jointly of the rails, the
earth itself, actual return wires which may supplement the rails, and
also all other conducting things in the earth, these being principally
lead-covered cables and other pipes. These conditions establish
definite areas in which the currents tend to leave the cables and
pipes, _i.e._, in which the latter are positive to other things. These
positive areas usually are much smaller than the negative areas, that
is, the regions in which currents tend _to enter_ the cables form a
larger total than the regions in which the currents tend _to leave_
the cables. These facts simplify the ways in which the cables may be
protected against damage by direct currents leaving them and also they
reduce the amount, complication, and cost of applying the corrective
and preventive measures.
All electric roads do not use direct current. Certain simplifications
in the use of single-phase alternating currents in traction motors
have increased the number of roads using a system of
alternating-current power supply. Where alternating current is used,
the electrolytic conditions are different and a new problem is set,
for, as the current flows in recurrently different directions, an area
which at one instant is positive to others, is changed the next
instant into a negative area. The protective means, therefore, must be
adapted to the changed requirements.
CHAPTER XIX
PROTECTIVE MEANS
Any of the heating hazards described in the foregoing chapter may
cause currents which will damage apparatus. All devices for the
protection of apparatus from such damage, operate either to stop the
flow of the dangerous current, or to send that flow over some other
path.

Conventional Symbols. In Fig. 82 are shown six conventional symbols
of polarized bells. The three at the top, consisting merely of two
circles representing the magnets in plan view, are perhaps to be
preferred as they are well standardized, easy to draw, and rather
suggestive. The three at the bottom, showing the ringer as a whole in
side elevation, are somewhat more specific, but are objectionable in
that they take more space and are not so easily drawn.
[Illustration: Fig. 82. Ringer Symbols]
Symbols _A_ or _B_ may be used for designating any ordinary polarized
ringer. Symbols _C_ and _D_ are interchangeably used to indicate a
biased ringer. If the bell is designed to operate only on positive
impulses, then the plus sign is placed opposite the symbol, while a
minus sign so placed indicates that the bell is to be operated only by
negative impulses.
Some specific types of ringers are designed to operate only on a given
frequency of current. That is, they are so designed as to be
responsive to currents having a frequency of sixty cycles per second,
for instance, and to be unresponsive to currents of any other
frequency. Either symbols _E_ or _F_ may be used to designate such
ringers, and if it is desired to indicate the particular frequency of
the ringer this is done by adding the proper numeral followed by a
short reversed curve sign indicating frequency. Thus 50~ would
indicate a frequency of fifty cycles per second.
CHAPTER IX
THE HOOK SWITCH
Purpose. In complete telephone instruments, comprising both talking
and signaling apparatus, it is obviously desirable that the two sets
of apparatus, for talking and signaling respectively, shall not be
connected with the line at the same time. A certain switching device
is, therefore, necessary in order that the signaling apparatus alone
may be left operatively connected with the line while the instrument
is not being used in the transmission of speech, and in order that the
signaling apparatus may be cut out when the talking apparatus is
brought into play.

One of these transmitters, embodying these same features but with
modified details, is shown in Fig. 42, this being the new transmitter
manufactured by the Western Electric Company. In this the bridge of
the original White transmitter is dispensed with, the electrode
chamber being supported by a pressed metal cup _1_, which supports the
chamber as a whole. The electrode cup, instead of being made of a
solid block as in the White instrument, is composed of two portions, a
cylindrical or tubular portion _2_ and a back _3_. The cylindrical
portion is externally screw-threaded so as to engage an internal screw
thread in a flanged opening in the center of the cup _1_. By this
means the electrode chamber is held in place in the cup _1_, and by
the same means the mica washer _4_ is clamped between the flange in
this opening and the tubular portion _2_ of the electrode chamber. The
front electrode is carried, as in the White transmitter, on the mica
washer and is rigidly attached to the center of the diaphragm so as to
partake of the movement thereof. It will be seen, therefore, that this
is essentially a White transmitter, but with a modified mounting for
the electrode chamber.
A feature in this transmitter that is not found in the White
transmitter is that both the front and the rear electrodes, in fact,
the entire working portions of the transmitter, are insulated from the
exposed metal parts of the instrument. This is accomplished by
insulating the diaphragm and the supporting cup _1_ from the
transmitter front. The terminal _5_ on the cup _1_ forms the
electrical connection for the rear electrode, while the terminal _6_,
which is mounted _on_ but insulated _from_ the cup _1_ and is
connected with the front electrode by a thin flexible connecting
strip, forms the electrical connection for the front electrode.

Bearing in mind that the calculations of Table IV are all based upon
the “diameter over insulation,” which it states at the outset for each
of four different kinds of covering, it is evident what is meant by
“turns per linear inch.” The columns referring to “turns per square
inch” mean the number of turns, the ends of which would be exposed in
one square inch if the wound coil were cut in a plane passing through
the axis of the core. Knowing the distance between the head, and the
depth to which the coil is to be wound, it is easy to select a size of
wire which will give the required number of turns in the provided
space. It is to be noted that the depth of winding space is one-half
of the difference between the core diameter and the complete diameter
of the wound coil. The resistance of the entire volume of wound wire
may be determined in advance by knowing the total cubic contents of
the winding space and multiplying this by the ohms per cubic inch of
the selected wire; that is, one must multiply in inches the distance
between the heads of the spool by the difference between the squares
of the diameters of the core and the winding space, and this in turn
by .7854. This result, times the ohms per cubic inch, as given in the
table, gives the resistance of the winding.
There is a considerable variation in the method of applying silk
insulation to the finer wires, and it is in the finer sizes that the
errors, if any, pile up most rapidly. Yet the table throughout is
based on data taken from many samples of actual coil winding by the
present process of winding small coils. It should be said further that
the table does not take into account the placing of any layers of
paper between the successive layers of the wires. This table has been
compared with many examples and has been used in calculating windings
in advance, and is found to be as close an approximation as is
afforded by any of the formulas on the subject, and with the further
advantage that it is not so cumbersome to apply.
_Winding Calculations._ In experimental work, involving the winding of
coils, it is frequently necessary to try one winding to determine its
effect in a given circuit arrangement, and from the knowledge so
gained to substitute another just fitted to the conditions. It is in
such a substitution that the table is of most value. Assume a case in
which are required a spool and core of a given size with a winding of,
say No. 25 single silk-covered wire, of a resistance of 50 ohms.
Assume also that the circuit regulations required that this spool
should be rewound so as to have a resistance of, say 1,000 ohms. What
size single silk-covered wire shall be used? Manifestly, the winding
space remains the same, or nearly so. The resistance is to be
increased from 50 to 1,000 ohms, or twenty times its first value.
Therefore, the wire to be used must show in the table twenty times as
many ohms per cubic inch as are shown in No. 25, the known first size.
This amount would be twenty times 7.489, which is 149.8, but there is
no size giving this exact resistance. No. 32, however, is very nearly
of that resistance and if wound to exactly the same depth would give
about 970 ohms. A few turns more would provide the additional thirty
ohms.
Similarly, in a coil known to possess a certain number of turns, the
table will give the size to be selected for rewinding to a greater or
smaller number of turns. In this case, as in the case of substituting
a winding of different resistance it is unnecessary to measure and
calculate upon the dimensions of the spool and core. Assume a spool
wound with No. 30 double silk-covered wire, which requires to be
wound with a size to double the number of turns. The exact size to do
this would have 8922. turns per square inch and would be between No.
34 and No. 35. A choice of these two wires may be made, using an
increased winding depth with the smaller wire and a shallower winding
depth for the larger wire.