In Fig. 185 is shown exactly the same arrangement, with the exception
that the talking apparatus illustrated in detail at Station A is that
of the Kellogg Switchboard and Supply Company. Otherwise the circuits
of the Dean and the Kellogg Company, and in fact of all the other
companies manufacturing harmonic ringing systems, are the same.
_Advantages_. A great advantage of the harmonic party-line system is
the simplicity of the apparatus at the subscribers station. The
harmonic bell is scarcely more complex than the ordinary polarized
ringer, and the only difference between the harmonic-ringing telephone
and the ordinary telephone is in the ringer itself. The absence of all
relays and other mechanism and also the absence of the necessity for
ground connections at the telephone are all points in favor of the
harmonic system.
[Illustration: Fig. 185. Circuits of Kellogg Harmonic System]
_Limitations_. As already stated, the harmonic systems of the various
companies, with one exception, are limited to four frequencies. The
exception is in the case of the North Electric Company, which sometimes
employs four and sometimes five frequencies and thus gets a selection
between five stations. In the four-party North system, the frequencies,
unlike those in the Dean and Kellogg systems, wherein the higher
frequencies are multiples of the lower, are arranged so as to be
proportional to the whole numbers 5, 7, 9, and 11, which, of course,
have no common denominator. The frequencies thus employed in the North
system are, in cycles per second, 30.3, 42.4, 54.5, and 66.7. In the
five-party system, the frequency of 16.7 is arbitrarily added.
While all of the commercial harmonic systems on the market are
limited to four or five frequencies, it does not follow that a greater
number than four or five stations may not be selectively rung. Double
these numbers may be placed on a party line and selectively actuated,
if the first set of four or five is bridged across the line and the
second set of four or five is connected between one limb of the line
and ground. The first set of these is selectively rung, as already
described, by sending the ringing currents over the metallic circuit,
while the second set may be likewise selectively rung by sending the
ringing currents over one limb of the line with a ground return. This
method is frequently employed with success on country lines, where it
is desired to place a greater number of instruments on a line than
four or five.
Step-by-Step Method. A very large number of step-by-step systems
have been proposed and reduced to practice, but as yet they have not
met with great success in commercial telephone work, and are nowhere
near as commonly used as are the polarity and harmonic systems.
_Principles_. An idea of the general features of the step-by-step
systems may be had by conceiving at each station on the line a ratchet
wheel, having a pawl adapted to drive it one step at a time, this pawl
being associated with the armature of an electromagnet which receives
current impulses from the line circuit. There is thus one of these
driving magnets at each station, each bridged across the line so that
when a single impulse of current is sent out from the central office
all of the ratchet wheels will be moved one step. Another impulse will
move all of the ratchet wheels another step, and so on throughout any
desired number of impulses. The ratchet wheels, therefore, are all
stepped in unison.

Series and Multiple Connections. When a number of voltaic cells are
joined in series, the positive pole of one being connected to the
negative pole of the next one, and so on throughout the series, the
_electromotive forces_ of all the cells are added, and the
electromotive force of the group, therefore, becomes the sum of the
electromotive forces of the component cells. The currents through all
the cells in this case will be equal to that of one cell.
If the cells be joined in multiple, the positive poles all being
connected by one wire and the negative poles by another, then the
_currents_ of all the cells will be added while the electromotive
force of the combination remains the same as that of a single cell,
assuming all the cells to be alike in electromotive force.
Obviously combinations of these two arrangements may be made, as by
forming strings of cells connected in series, and connecting the
strings in multiple or parallel.
The term battery is frequently applied to a single voltaic cell, but
this term is more properly used to designate a plurality of cells
joined together in series, or in multiple, or in series multiple so as
to combine their actions in causing current to flow through an
external circuit. We may therefore refer to a battery of so many
cells. It has, however, become common, though technically improper, to
refer to a single cell as a battery, so that the term battery, as
indicating necessarily more than one cell, has largely lost its
significance.
Cells may be of two types, primary and secondary.
Primary cells are those consisting of electrodes of dissimilar
elements which, when placed in an electrolyte, become immediately
ready for action.
Secondary cells, commonly called _storage cells_ and _accumulators_,
consist always of two inert plates of metal, or metallic oxide,
immersed in an electrolyte which is incapable of acting on either of
them until a current has first been passed through the electrolyte
from one plate to the other. On the passage of a current in this way,
the decomposition of the electrolyte is effected and the composition
of the plates is so changed that one of them becomes electro-positive
and the other electro-negative. The cell is then, when the _charging_
current ceases, capable of acting as a voltaic cell.
This chapter is devoted to the primary cell or battery alone.
Types of Primary Cells. Primary cells may be divided into two
general classes: first, those adapted to furnish constant current; and
second, those adapted to furnish only intermittent currents. The
difference between cells in this respect rests largely in the means
employed for preventing or lessening polarization. Obviously in a cell
in which polarization is entirely prevented the current may be allowed
to flow constantly until the cell is completely exhausted; that is,
until the zinc is all eaten up or until the hydrogen is exhausted from
the electrolyte or both. On the other hand some cells are so
constituted that polarization takes place faster than the means
intended to prevent it can act. In other words, the polarization
gradually gains on the preventive means and so gradually reduces the
current by increasing the resistance of the cell and lowering its
electromotive force. In cells of this kind, however, the arrangement
is such that if the cell is allowed to rest, that is, if the external
circuit is opened, the depolarizing agency will gradually act to
remove the hydrogen from the unattacked electrode and thus place the
cell in good condition for use again.
Of these two types of primary cells the intermittent-current cell is
of far greater use in telephony than the constant-current cell. This
is because the use of primary batteries in telephony is, in the great
majority of cases, intermittent, and for that reason a cell which will
give a strong current for a few minutes and which after such use will
regain practically all of its initial strength and be ready for use
again, is more desirable than one which will give a weaker current
continuously throughout a long period of time.
Since the cells which are adapted to give constant current are
commonly used in connection with circuits that are continuously
closed, they are called _closed-circuit cells_. The other cells, which
are better adapted for intermittent current, are commonly used on
circuits which stand open most of the time and are closed only
occasionally when their current is desired. For this reason these are
termed _open-circuit cells_.
_Open-Circuit Cells_. LeClanché Cell:–By far the most important
primary cell for telephone work is the so-called LeClanché cell. This
assumes a large variety of forms, but always employs zinc as the
negatively charged element, carbon as the positively charged element,
and a solution of sal ammoniac as the electrolyte. This cell employs a
chemical method of taking care of polarization, the depolarizing agent
being peroxide of manganese, which is closely associated with the
carbon element.

A complete K.B. lock-out telephone is shown in Fig. 190. This is the
type of instrument that is usually furnished when new equipment is
ordered. If, however, it is desired to use the K.B. system in
connection with telephones of the ordinary bridging type that are
already in service, the lock-out and selective mechanism, which is
shown on the upper inner face of the door in Fig. 190, is furnished
separately in a box that may be mounted close to the regular telephone
and connected thereto by suitable wires, as shown in Fig. 191. It is
seen that this instrument employs a local battery for talking and also
a magneto generator for calling the central office.
The central-office equipment consists of a dial connected with an
impulse wheel, together with suitable keys by which the various
circuits may be manipulated. This dial and its associated mechanism
may be mounted in the regular switchboard cabinet, or it may be
furnished in a separate box and mounted alongside of the cabinet in
either of the positions shown at _1_ or _2_ of Fig. 192.
In order to send the proper number of impulses to the line to call a
given party, the operator places her finger in the hole in the dial
that bears the number corresponding to the station wanted and rotates
the dial until the finger is brought into engagement with the fixed
stop shown at the bottom of the dial in Fig. 192. The dial is then
allowed to return by the action of a spring to its normal position,
and in doing so it operates a switch within the box to make and break
the battery circuit the proper number of times.
_Operation._ A complete description of the operation may now be had in
connection with Fig. 193, which is similar to Fig. 189, but contains
the details of the calling arrangement at the central office and also
of the talking circuits at the various subscribers stations.
[Illustration: Fig. 191. K.B. Lock-Out Station]

Great use is made in the design of telephone circuits of the fact that
the electromagnets, which accomplish the useful mechanical results in
causing the movement of parts, possess the quality of impedance. Thus,
the magnets which operate various signaling relays at the central
office are often used also as impedance coils in portions of the
circuit through which it is desired to have only steady currents pass.
If, on the other hand, it is necessary to place a relay magnet, having
considerable impedance, directly in a talking circuit, the bad effects
of this on the voice currents may be eliminated by shunting this coil
with a condenser, or with a comparatively high non-inductive
resistance. The voice currents will flow around the high impedance of
the relay coil through the condenser or resistance, while the steady
currents, which are the ones which must be depended upon to operate
the relay, are still forced in whole or in part to pass through the
relay coil where they belong.
In a similar way the induction coil affords a means for keeping two
circuits completely isolated so far as the direct flow of current
between them is concerned, and yet of readily transmitting, by
electromagnetic induction, currents from one of these circuits to the
other. Here is a means of isolation so far as direct current is
concerned, with complete communication for alternating current.
CHAPTER XIII
CURRENT SUPPLY TO TRANSMITTERS
The methods by which current is supplied to the transmitter of a
telephone for energizing it, may be classified under two divisions:
first, those where the battery or other source of current is located
at the station with the transmitter which it supplies; and second,
those where the battery or other source of current is located at a
distant point from the transmitter, the battery in such cases serving
as a common source of current for the supply of transmitters at a
number of stations.
The advantages of putting the transmitter and the battery which
supplies it with current in a local circuit with the primary of an
induction coil, and placing the secondary of the induction coil in the
line, have already been pointed out but may be briefly summarized as
follows: When the transmitter is placed directly in the _line circuit_
and the line is of considerable length, the current which passes
through the transmitter is necessarily rather small unless a battery
of high potential is used; and, furthermore, the total change in
resistance which the transmitter is capable of producing is but a
small proportion of the total resistance of the line, and, therefore,
the current changes produced by the transmitter are relatively small.
On the other hand, when the transmitter is placed in a _local circuit_
with the battery, this circuit may be of small resistance and the
current relatively large, even though supplied by a low-voltage
battery; so that the transmitter is capable of producing relatively
large changes in a relatively large current.
To draw a comparison between these two general classes of transmitter
current supply, a number of cases will be considered in connection
with the following figures, in each of which two stations connected by
a telephone line are shown. Brief reference to the local battery
method of supplying current will be made in order to make this chapter
contain, as far as possible, all of the commonly used methods of
current supply to transmitters.
[Illustration: A TYPICAL MEDIUM-SIZED MULTIPLE SWITCHBOARD EQUIPMENT]
Local Battery. In Fig. 125 two stations are shown connected by a
grounded line wire. The transmitter of each station is included in a
low-resistance primary circuit including a battery and the primary
winding of an induction coil, the relation between the primary
circuits and the line circuits being established by the inductive
action between the primary and the secondary windings of induction
coils, the secondary in each case being in the line circuits with the
receivers.
[Illustration: Fig. 125. Local-Battery Stations with Grounded Circuit]

TABLE V
Temperature Coefficients
+—————————+—————————–+
| PURE METALS | TEMPERATURE COEFFICIENTS |
+—————————+————–+————–+
| | CENTIGRADE | FAHRENHEIT |
+—————————+————–+————–+
| Silver (annealed) | 0.00400 | 0.00222 |
| Copper (annealed) | 0.00428 | 0.00242 |
| Gold (99.9%) | 0.00377 | 0.00210 |
| Aluminum (99%) | 0.00423 | 0.00235 |
| Zinc | 0.00406 | 0.00226 |
| Platinum (annealed) | 0.00247 | 0.00137 |
| Iron | 0.00625 | 0.00347 |
| Nickel | 0.0062 | 0.00345 |
| Tin | 0.00440 | 0.00245 |
| Lead | 0.00411 | 0.00228 |
| Antimony | 0.00389 | 0.00216 |
| Mercury | 0.00072 | 0.00044 |
| Bismuth | 0.00354 | 0.00197 |
+—————————+————–+————–+
_Positive and Negative Coefficients._ Those conductors, in which a
rise in temperature produces an increase in resistance, are said to
have positive temperature coefficients, while those in which a rise in
temperature produces a lowering of resistance are said to have
negative temperature coefficients.
The temperature coefficients of pure metals are always positive and
for some of the more familiar metals, have values, according to
Foster, as in Table V.
Iron, it will be noticed, has the highest temperature coefficient of
all. Carbon, on the other hand, has a large negative coefficient, as
proved by the fact that the filament of an ordinary incandescent lamp
has nearly twice the resistance when cold as when heated to full
candle-power.
Certain alloys have been produced which have very low temperature
coefficients, and these are of value in producing resistance units
which have practically the same resistance for all ordinary
temperatures. Some of these alloys also have very high resistance as
compared with copper and are of value in enabling one to obtain a high
resistance in small space.

This defect may in some measure be reduced by making the ringers of
low impedance. This is the general practice with series telephones,
the ringers ordinarily having short cores and a comparatively small
number of turns, the resistance being as a rule about 80 ohms.
Bridging Systems. Very much better than the series plan of
party-line connections, is the arrangement by which the instruments
are placed in bridges across the line, such lines being commonly known
as bridged or bridging lines. This was first strongly advocated and
put into wide practical use by J.J. Carty, now the Chief Engineer of
the American Telephone and Telegraph Company.
A simple illustration of a bridging telephone line is shown in Fig.
166, where the three telephones shown are each connected in a bridge
path from the line wire to ground, a type known as a “grounded
bridging line.” Its use is very common in rural districts.
A better arrangement is shown in Fig. 167, which represents a
metallic-circuit bridging line, three telephone instruments being
shown in parallel or bridge paths across the two line wires.
The actual circuit arrangements of a bridging party line are better
shown in Fig. 168. There are three stations and it will be seen that
at each station there are three possible bridges, or bridge paths,
across the two limbs of the line. The first of these bridges is
controlled by the hook switch and is normally open. When the hook is
raised, however, this path is closed through the receiver and
secondary of the induction coil, the primary circuit being also closed
so as to include the battery and transmitter. This constitutes an
ordinary local-battery talking set.
[Illustration: Fig. 166. Grounded Bridging Line]
[Illustration: Fig. 167. Metallic Bridging Line]
[Illustration: Fig. 168. Metallic Bridging Line]
A second bridge at each station is led through the ringer or
call-bell, and this, in most bridging telephones, is permanently
closed, the continuity of this path between the two limbs of the line
not being affected either by the hook switch or by the automatic
switch in connection with the generator.
A third bridge path at each station is led through the generator.
This, as indicated, is normally open, but the automatic cut-in switch
of the generator serves, when the generator is operated, to close its
path across the line, so that it may send its currents to the line and
ring the bells of all the stations.
When any generator is operated, its current divides and passes over
the line wires and through all of the ringers in multiple. It is seen,
therefore, that the requirements for a bridging generator are that it
shall be capable of generating a large current, sufficient when
divided up amongst all the bells to ring each of them; and that it
shall be capable of producing a sufficient voltage to send the
required current not only to the near-by stations, but to the stations
at the distant end of the line.
It might seem at first that the bridging system avoided one difficulty
only to encounter another. It clearly avoids the difficulty of the
series system in that the voice currents, in order to reach distant
stations, do not have to pass through all of the bells of the idle
stations in series. There is, however, presented at each station a
leakage path through the bell bridged across the line, through which
it would appear the voice currents might leak uselessly from one side
of the line to the other and not pass on in sufficient volume to the
distant station. This difficulty is, however, more apparent than real.
It is found that, by making the ringers of high impedance, the leakage
of voice currents through them from one side of the line to the other
is practically negligible.
It is obvious that in a heavily loaded bridged line, the bell at the
home station, that is at the station from which the call is being sent,
will take slightly more than its share of the current, and it is also
obvious that the ringing of the home bell performs no useful function.
The plan is frequently adopted, therefore, of having the operation of
the generator serve to cut its own bell out of the circuit. The
arrangement by which this is done is clearly shown in Fig. 169. The
circuit of the bell is normally complete across the line, while the
circuit of the generator is normally open. When, however, the generator
crank is turned these conditions are reversed, the bell circuit being
broken and the generator circuit closed, so as to allow its current all
to pass the line. This feature of having the local bell remain silent
upon the operation of its own generator is also of advantage because
other parties at the same station are not disturbed by the ringing of
the bell when a call is being made by that station.

Magnet Wire. The wire used in winding magnets is, of course, an
important part of the electromagnet. It is always necessary that the
adjacent turns of the wire be insulated from each other so that the
current shall be forced to pass around the core through all the length
of wire in each turn rather than allowing it to take the shorter and
easier path from one turn to the next, as would be the case if the
turns were not insulated. For this purpose the wire is usually covered
with a coating of some insulating material. There are, however,
methods of winding magnet coils with bare wire and taking care of the
insulation between the turns in another way, as will be pointed out.
Insulated wire for the purpose of winding magnet coils is termed
_magnet wire_. Copper is the material almost universally employed for
the conductor. Its high conductivity, great ductility, and low cost
are the factors which make it superior to all other metals. However,
in special cases, where exceedingly high conductivity is required with
a limited winding space, silver wire is sometimes employed, and on the
other hand, where very high resistance is desired within a limited
winding space either iron or German silver or some other
high-resistance alloy is used.
_Wire Gauges_. Wire for electrical purposes is drawn to a number of
different standard gauges. Each of the so-called wire gauges consists
of a series of graded sizes of wire, ranging from approximately
one-half an inch in diameter down to about the fineness of a ladys
hair. In certain branches of telephone work, such as line
construction, the existence of the several wire gauges or standards is
very likely to lead to confusion. Fortunately, however, so far as
magnet wire is concerned, the so-called Brown and Sharpe, or American,
wire gauge is almost universally employed in this country. The
abbreviations for this gauge are B.&S. or A.W.G.

Lalande Cell:–A type of cell, specially adapted to constant-current
work, and sometimes used as a central source of current in very small
common-battery exchanges is the so-called _copper oxide_, or _Lalande
cell_, of which the Edison and the Gordon are types. In all of these
the negatively charged element is of zinc, the positively charged
element a mass of copper oxide, and the electrolyte a solution of
caustic potash in water. In the Edison cell the copper oxide is in the
form of a compressed slab which with its connecting copper support
forms the electrode. In the Gordon and other cells of this type the
copper oxide is contained loosely in a perforated cylinder of sheet
copper. The copper oxide serves not only as an electrode, but also as
a depolarizing agent, the liberated hydrogen in the electrolyte
uniting with the oxygen of the copper oxide to form water, and leaving
free metallic copper.
On open circuit the elements are not attacked, therefore there is no
waste of material while the cell is not in use. This important
feature, and the fact that the internal resistance is low, make this
cell well adapted for all forms of heavy open-circuit work. The fact
that there is no polarizing action within the cell makes it further
adaptable to heavy closed-circuit service.
These cells are intended to be so proportioned that all of their parts
become exhausted at once so that when the cell fails, complete
renewals are necessary. Therefore, there is never a question as to
which of the elements should be renewed
After the elements and solution are in place about one-fourth of an
inch of heavy paraffin oil is poured upon the surface of the solution
in order to prevent evaporation. This cell requires little attention
and will maintain a constant e.m.f. of about two-thirds of a volt
until completely exhausted. It is non-freezable at all ordinary
temperatures. Its low voltage is its principal disadvantage.
_Standard Cell_. Chloride of Silver Cell:–The chloride of silver cell
is largely used as a standard for testing purposes. Its compactness
and portability and its freedom from local action make it particularly
adaptable to use in portable testing outfits where constant
electromotive force and very small currents are required.
[Illustration: Fig. 66. Chloride of Silver Cell]
A cross-section of one form of the cell is shown in Fig. 66. Its
elements are a rod of chemically-pure zinc and a rod of chloride of
silver immersed in a water solution of sal ammoniac. As ordinarily
constructed, the glass jar or tube is usually about 2-1/2 inches long
by 1 inch in diameter. After the solution is poured in and the
elements are in place the glass tube is hermetically sealed with a
plug of paraffin wax.

The diagram of Fig. 68 is merely intended to illustrate the principle
involved. In the practical construction of magneto generators more
than one bar magnet is used, and, in addition, the conductors in the
armature are so arranged as to include a great many loops of wire.
Furthermore, the conductors in the armature are wound around an iron
core so that the path through the armature loops or turns, may present
such low reluctance to the passage of lines of force as to greatly
increase the number of such lines and also to cause practically all of
them to go through the loops in the armature conductor.
Armature. The iron upon which the armature conductors are wound is
called the _core_. The core of an ordinary armature is shown in Fig.
69. This is usually made of soft gray cast iron, turned so as to form
bearing surfaces at _1_ and _2_, upon which the entire armature may
rotate, and also turned so that the surfaces _3_ will be truly
cylindrical with respect to the axis through the center of the shaft.
The armature conductors are put on by winding the space between the
two parallel faces _4_ as full of insulated wire as space will admit.
One end of the armature winding is soldered to the pin _5_ and,
therefore, makes contact with the frame of the generator, while the
other end of the winding is soldered to the pin _6_, which engages the
stud _7_, carried in an insulating bushing in a longitudinal hole in
the end of the armature shaft. It is thus seen that the frame of the
machine will form one terminal of the armature winding, while the
insulated stud _7_ will form the other terminal.
[Illustration: Fig. 69. Generator Armature]
Another form of armature largely employed in recent magneto
generators is illustrated in Fig. 70. In this the shaft on which the
armature revolves does not form an integral part of the armature core
but consists of two cylindrical studs _2_ and _3_ projecting from the
centers of disks _4_ and _5_, which are screwed to the ends of the
core _1_. This =H= type of armature core, as it is called, while
containing somewhat more parts than the simpler type shown in Fig. 69,
possesses distinct advantages in the matter of winding. By virtue of
its simpler form of winding space, it is easier to insulate and easier
to wind, and furthermore, since the shaft does not run through the
winding space, it is capable of holding a considerably greater number
of turns of wire. The ends of the armature winding are connected, one
directly to the frame and the other to an insulated pin, as is shown
in the illustration.
[Illustration: Fig. 70. Generator Armature]
[Illustration: Fig. 71. Generator Field and Armature]
The method commonly employed of associating the pole pieces with each
other and with the permanent magnets is shown in Fig. 71. It is very
important that the space in which the armature revolves shall be truly
cylindrical, and that the bearings for the armature shall be so
aligned as to make the axis of rotation of the armature coincide with
the axis of the cylindrical surface of the pole pieces. A rigid
structure is, therefore, required and this is frequently secured, as
shown in Fig. 71, by joining the two pole pieces _1_ and _2_ together
by means of heavy brass rods _3_ and _4_, the rods being shouldered
and their reduced ends passed through holes in flanges extending from
the pole pieces, and riveted. The bearing plates in which the armature
is journaled are then secured to the ends of these pole pieces, as
will be shown in subsequent illustrations. This assures proper
rigidity between the pole pieces and also between the pole pieces and
the armature bearings.
The reason why this degree of rigidity is required is that it is
necessary to work with very small air gaps between the armature core
and its pole pieces and unless these generators are mechanically well
made they are likely to alter their adjustment and thus allow the
armature faces to scrape or rub against the pole pieces. In Fig. 71
one of the permanent horseshoe magnets is shown, its ends resting in
grooves on the outer faces of the pole pieces and usually clamped
thereto by means of heavy iron machine screws.
With this structure in mind, the theory of the magneto generator
developed in connection with Fig. 68 may be carried a little further.
When the armature lies in the position shown at the left of Fig. 71,
so that the center position of the core is horizontal, a good path is
afforded for the lines of force passing from one pole to the other.

The position of the deepest notch, _i.e._, the selective notch, on the
circumference of the segment at any station depends upon the number of
that station; thus, the segment of Station 4 will have a deep notch in
the sixth position; the segment for Station 9 will have a deep notch
in the eleventh position; the segment for any station will have a deep
notch in the position corresponding to the number of that station plus
two.
From what has been said, therefore, it is evident that the first, or
normal, notch on each segment is of such a depth as to allow the
moving pawl _6_ to fall to such a depth in the segment as to permit
the rocker arm _2_ to close the talking circuit only. All of the other
notches, except one, are comparatively shallow, and while they permit
the moving pawl _6_ under the influence of the rocker arm _2_ to move
the segment _3_, yet they do not permit the rocker arm _2_ to move so
far to the left as to close even the talking circuit. The exception is
the deep notch, or selective notch, which is of such depth as to
permit the pawl _6_ to fall so far into the segment as to allow the
rocker arm _2_ to close both the talking and the ringing circuits.
Besides the moving pawl _6_ there is a detent pawl _7_. This always
holds the segment _3_ in the position to which it has been last moved
by the moving pawl _6_.
The actuating magnet _1_, as has been stated, is polarized and when
energized by currents in one direction, the rocker arm moves the pawl
_6_ so as to step the segment one notch. When this relay is energized
by current in the opposite direction, the operation is such that both
the moving pawl _6_ and the detent pawl _7_ will be pulled away from
the segment, thus allowing the segment to return to its normal position
by gravity. This is accomplished by the following mechanism: An
armature stop is pivoted upon the face of the rocker arm so as to swing
in a plane parallel to the pole faces of the relay, and is adapted,
when the relay is actuated by selective impulses of one polarity, to be
pulled towards one of the pole faces where it acts, through impact with
a plate attached to the pole face of the relay, as a limiting means
for the motion of the rocker arm when the rocker arm is actuated by the
magnet. When, however, the relay is energized by current in the
opposite direction, as on a releasing impulse, the armature stop swings
upon its pivot towards the opposite pole face, in which position the
lug on the end of the armature stop registers with a hole in the plate
on the relay, thus allowing the full motion of the rocker arm when it
is attracted by the magnet. This motion of the rocker arm withdraws the
detent pawl from engagement with the segment as well as the moving
pawl, and thereby permits the segment to return to its normal position.
As will be seen from Fig. 189, each of the relay magnets _1_ is
permanently bridged across the two limbs of the line.
Each station is provided with a push button, not shown, by means of
which the subscriber who makes a call may prevent the rocker arm of
his instrument from being actuated while selective impulses are being
sent over the line. The purpose of this is to enable one party to make
a call for another on the same line, depressing his push button while
the operator is selecting and ringing the called party. The segment at
his own station, therefore, remains in its normal position, in which
position, as we have already seen, his talking circuit is closed; all
of the other segments are, however, stepped up until the ringing and
talking circuits of the desired station are in proper position, at
which time ringing current is sent over the line. The segments in Fig.
189, except at Station C, are shown as having been stepped up to the
sixth position, which corresponds to the ringing position of the
fourth station, or Station D. The condition shown in this figure
corresponds to that in which the subscriber at Station C originated
the call and pressed his button, thus retaining his own segment in its
normal position so that the talking circuits would be established with
Station D.
When the line is in normal position any subscriber may call central by
his magneto generator, not shown in Fig. 189, which will operate the
drop at central, but will not operate any of the subscribers bells,
because all bell circuits are normally open. When a subscriber desires
connection with another line, the operator sends an impulse back on
the line which steps up and locks out all instruments except that of
the calling subscriber.
[Illustration: Fig. 190. K.B. Lock-Out Station]