THE COMMON-BATTERY MULTIPLE SWITCHBOARD
=Western Electric No. 1 Relay Board.= The common-battery multiple
switchboard differs from the simple or non-multiple common-battery
switchboard mainly in the provision of multiple jacks and in the added
features which are involved in the provision for a busy test. The
principles of signaling and of supplying current to the subscribers for
talking are the same as in the non-multiple common-battery board. For
purposes of illustrating the practical workings of the common-battery
multiple switchboard, we will take the standard form of the Western
Electric Company, choosing this only because it is the standard with
nearly all the Bell operating companies throughout the United States.
[Illustration: Fig. 345. Line Circuit Western Electric No. 1. Board]
_Line Circuit._ We will first consider the line circuit in simplified
form, as shown in Fig. 345. At the left in this figure the
common-battery circuit is shown at the subscriber's station, and at the
right the central-office apparatus is indicated so far as equipment of a
single line is concerned. In this simplified diagram no attempt has been
made to show the relative positions of the various parts, these having
been grouped in this figure in such a way as to give as clear and simple
an idea as possible of the circuit arrangements. It is seen at a glance
that this is a branch terminal board, the three contacts of each jack
being connected by separate taps or legs to three wires running
throughout the length of the board, these three wires being individual
to the jacks of one line. On this account this line circuit is commonly
referred to as a three-wire circuit. By the same considerations it will
be seen that the switchboard line circuit of the branch-terminal
multiple magneto system, shown in Fig. 338, would be called a four-wire
circuit. It will be shown later that other multiple switchboards in wide
use have a still further reduction in the number of wires running
through the jacks, or through the multiple as it is called, such being
referred to as two-wire switchboards.
The two limbs of the line which extend from the subscriber's circuit,
beside being connected by taps to the tip and sleeve contacts of the
jack respectively, connect with the two back contacts of a cut-off
relay, and when this relay is in its normal or unenergized condition,
these two limbs of the line are continued through the windings of the
line relay and thence one to the ungrounded or negative side of the
common-battery and the other to the grounded side. The subscriber's
station circuit being normally open, no current flows through the line,
but when the subscriber removes his receiver for the purpose of making a
call the line circuit is completed and current flows through the coil of
the line relay, thus energizing that relay and causing it to complete
the circuit of the line lamp. The cut-off relay plays no part in the
operation of the subscriber's calling, but merely leaves the circuit of
the line connected through to the calling relay and battery. The coil of
the cut-off relay is connected to ground on one side and on the other
side to the third wire running through the switchboard multiple and
which is tapped off to each of the test rings on the jacks. As will be
shown later, when the operator plugs into the jack of a line, such a
connection is established that the test ring of that jack will be
connected to the live or negative pole of the common battery, which will
cause current to flow through the coil of the cut-off relay, which will
then operate to _cut off_ both of the limbs of the line from their
normal connection with ground and the battery and the line relay. Hence
the name _cut-off relay_.
The use of the cut-off relay to sever the calling apparatus from the
line at all times when the line is switched serves to make possible a
very much simpler jack than would otherwise be required, as will be
obvious to anyone who tries to design a common-battery multiple system
without a cut-off relay. The additional complication introduced by the
cut-off relay is more than offset by the saving in complexity of the
jacks. It is desirable, on account of the great number of jacks
necessarily employed in a multiple switchboard, that the jacks be of the
simplest possible construction, thus reducing to a minimum their first
cost and making them much less likely to get out of order.
_Cord Circuit._ The cord circuit of the Western Electric standard
multiple common-battery switchboard is shown in Fig. 346. This cord
circuit involves the use of three strands in the flexible cords of both
the calling and the answering plugs. Two of these are the ordinary tip
and ring conductors over which speech is transmitted to the connected
subscriber's wire. The third, the sleeve strand, carries the supervisory
lamps and has associated with it other apparatus for the control of
these lamps and of the test circuit.
[Illustration: Fig. 346. Cord Circuit Western Electric No. 1 Board]
The system of battery feed is the well-known split repeating-coil
arrangement already discussed. The tip strand runs straight through to
the repeating coil, while the ring strand contains, in each case, the
winding of the supervisory relay corresponding to either the calling or
the answering plug. In order that the presence in the talking circuit of
a magnet winding possessing considerable impedance may not interfere
with the talking efficiency, each of these supervisory relay windings is
shunted by a non-inductive resistance. In practice the supervisory relay
windings have each a resistance of about 20 ohms and the shunt around
them each a resistance of about 31 ohms. In the third strand of each
cord is placed a 12-volt supervisory lamp, and in series with it a
resistance of about 80 ohms. Each supervisory relay is adapted, when
energized, to close a 40-ohm shunt about its supervisory lamp. The
arrangement and proportion of these resistances is such that when a plug
is inserted into the jack of a line the lamp will receive current from a
circuit traced from the negative pole of the battery in the center of
the cord circuit through the lamp and the 80-ohm series resistance,
through the third strand of the cord to the test thimble of the jack,
and thence to the positive or grounded pole of the battery through the
third conductor in the multiple and the winding of the cut-off relay.
This current always flows as long as the plug is inserted, and it is
just sufficient to illuminate the lamp when the supervisory relay
armature is not attracted. When, however, the supervisory relay armature
is attracted, the shunting of the lamp by the 40-ohm resistance cuts
down the current to such a degree as to prevent the illumination of the
lamp, although some current still flows through it.
The usual ringing and listening key is associated with the calling plug,
and in some cases a ring-back key is associated with the answering plug,
but this is not standard practice.
_Operation._ The operation of this cord circuit in conjunction with the
line circuit of Fig. 345 may best be understood by reference to Fig.
347. This figure employs a little different arrangement of the line
circuit in order more clearly to indicate how the two lines may be
connected by a cord; a study of the two line circuits, however, will
show that they are identical in actual connections. It is to be
remembered that all of the battery symbols shown in this figure
represent in reality the same battery, separate symbols being shown for
greater simplicity in circuit connections.
We will assume the subscriber at Station _A_ calls for the subscriber at
Station _B_. The operation of the line relay and the consequent lighting
of the line lamp, and also the operation of the pilot relay will be
obvious from what has been stated. The response of the operator by
inserting the answering plug into the answering jack, and the throwing
of her listening key so as to bridge her talking circuit across the cord
in order to place herself in communication with the subscriber, is also
obvious. The insertion of the answering plug into the answering jack
completed the circuit through the third strand of the cord and the
winding of the cut-off relay of the calling line, and this accomplishes
three desirable results. The circuit so completed may be traced from the
negative or ungrounded side of the battery to the center portion of the
cord circuit, thence through the supervisory lamp _1_, resistance _2_,
to the third conductor on the plug, test thimble on the jack, thence
through the winding of the cut-off relay to ground, which forms the
other terminal of the battery. The results accomplished by the closing
of this circuit are: first, the energizing of the cut-off relay to cut
off the signaling portion of the line; second, the flowing of current
through the lamp that is almost sufficient to illuminate it, but not
quite so because of the closure of the shunt about it, for the reason
that will be described; third, the raising of the potential of all the
contact thimbles on the jacks from zero to a potential different from
that of the ground and equal in amount to the fall of potential through
the winding of the cut-off relay. A condition is thus established at the
test rings such that some other operator at some other section in
testing the line will find it busy and will not connect with it.
[Illustration: Fig. 347. Western Electric No. 1 Board]
The reason why the lamp _1_, connected with the answering plug, was not
lighted was that the supervisory relay _3_, associated with the
answering plug, became energized when the operator plugged in, due to
the flow of current from the battery through the calling subscriber's
talking apparatus, this flow of current being permitted by the removal
of the calling subscriber's receiver from its hook. The energizing of
this relay magnet by causing the attraction of its armature, closed the
shunt about the lamp _1_, which shunt contains the 40-ohm resistance
_4_, and thus prevents the lamp from receiving enough current to
illuminate it. Obviously, as soon as the calling subscriber replaces his
receiver on its hook, the supervisory relay _3_ will be de-energized,
the shunt around the lamp will be broken, and the lamp will be
illuminated to indicate to the operator the fact that the subscriber
with whose line her calling plug is connected has replaced his receiver
on its hook.
_Testing--Called Line Idle._ Having now shown how the operator connects
with the calling subscriber's line and how that line automatically
becomes guarded as soon as it is connected with, so that no other
operator will connect with it, we will discuss how the operator tests
the called line and subsequently connects with that line, if it is found
proper to do so. If, on making the test with one of the multiple jacks
of the line leading to Station _B_, that line is idle and free to be
connected with, its test rings will all be at zero potential because of
the fact that they are connected with ground through the cut-off relay
winding with no source of current connected with them. The tip of the
calling plug will also be at zero potential in making this test, because
it is connected to ground through the tip side of the calling-plug
circuit and one winding of the cord-circuit repeating coil. As a result
no flow of current will occur, the operator will receive no click, and
she will know that she is free to connect with the line. As soon as she
does so, by inserting the plug, the third strand of the cord will be
connected with the test thimble of the calling line and the resulting
flow of current will bring about three results, two of which are the
same, and one of which is slightly different from those described as
resulting from the insertion of the answering plug into the jack of the
calling line. First, the cut-off relay will be operated and cut off the
line signaling apparatus from the called line; second, a flow of current
will result through the calling supervisory lamp _5_, which in this case
will be sufficient to illuminate that lamp for the reason that the
called subscriber has not yet responded, the calling supervisory relay
_6_ has, therefore, not yet been energized, and the lamp has not,
therefore, been shunted by its associated resistance _7_; third, the
test thimbles of the called line will be raised to a potential above
that of the earth, and thus the line will be guarded against connection
at another section of the switchboard. As soon as the called subscriber
responds to the ringing current sent out by the operator, current will
flow over the cord circuit and over his line through his transmitter.
This will cause the calling supervisory relay to be energized and the
calling lamp to be extinguished. Both lamps _1_ and _5_ remain
extinguished as long as the connected subscribers are in conversation,
but as soon as either one of them hangs up his receiver the
corresponding lamp will be lighted, due to the de-energization of the
supervisory relay and the breaking of the shunt around the lamp. The
lighting of both lamps associated with a cord circuit is a signal to the
operator for disconnection.
[Illustration: TERMINAL ROOM IN MEDIUM-SIZED MANUAL OFFICE Relay Rack at
Right. This Employs the Kellogg Parallel Arrangement of Frames.]
_Testing--Called Line Busy._ If we now assume that the called line was
already busy, by virtue of being connected with at another section, the
test rings of that line would accordingly all be raised to a potential
above that of the earth. As a result, when the operator applied the tip
of her calling plug to a test thimble on that line, current would flow
from this test thimble through the tip of the calling plug and tip
strand of the cord and through one winding of the cord-circuit repeating
coil to ground. This would cause a slight raising of potential of the
entire tip side of the cord circuit and a consequent momentary flow of
current through the secondary of the operator's circuit bridged across
the cord circuit at that time.
_Operator's Circuit Details._ The details of the operator's talking
circuit shown in Fig. 347 deserve some attention. The battery supply to
the operator's transmitter is through an impedance coil _9_. The
condenser _12_ is bridged around the transmitter and the two primary
windings _10_ and _11_, which windings are in parallel so as to afford a
local circuit for the passage of fluctuating currents set up by the
transmitter. The two primary windings _10_ and _11_ are on separate
induction coils, the secondary windings _13_ and _14_ being, therefore,
on separate cores. The winding _15_, in circuit with the secondary
winding _14_ and the receiver, is a non-inductive winding and is
supposed to have a resistance about equal to the effective resistance to
fluctuating currents of a subscriber's line of average length. Owing to
the respective directions of the primary and secondary windings _10_ and
_11_, _13_ and _14_, the result is that the outgoing currents set up by
the operator's transmitter are largely neutralized in the operator's
receiver. Incoming currents from either of the connected subscribers,
however, pass, in the main, through the secondary coil _13_ and the
operator's receiver, rather than through the shunt path formed by the
secondary _14_, and the non-inductive resistance _15_. This is known as
an "anti-side tone" arrangement, and its object is to prevent the
operator from receiving her own voice transmission so loudly as to make
her ear insensitive to the feebler voice currents coming in from the
subscribers.
_Order-Wire Circuits._ The two keys _16_ and _17_, shown in connection
with the operator's talking circuit in Fig. 347, play no part in the
regular operation of connecting two local lines, as described above.
They are order-wire keys, and the circuits with which they connect lead
to the telephone sets of other operators at distant central offices, and
by pressing either one of these keys the operator is enabled to place
herself in communication over these so-called order-wire circuits with
such other operators. The function and mode of operation of these
order-wire circuits will be described in the next chapter, wherein
inter-office connections will be discussed.
_Wiring of Line Circuit._ The line circuits shown in Figs. 345 and 347
are, as stated, simplified to facilitate understanding, although the
connections shown are those which actually exist. The more complete
wiring of a single line circuit is shown in Fig. 348. The line wires are
shown entering at the left. They pass immediately, upon entering the
central office, through the main distributing frame, the functions and
construction of which will be considered in detail in a subsequent
chapter. The dotted portions of the circuit shown in connection with
this main distributing frame indicate the path from the terminals on one
side of the frame to those on the other through so-called jumper wires.
The two limbs of the line then pass to terminals _1_ and _2_ on one side
of the so-called intermediate distributing frame. Here the circuit of
each limb of the line divides, passing, on the one hand, to the tip and
sleeve springs of all the multiple jacks belonging to that line; and, on
the other hand, through the jumper wires indicated by dotted lines on
the intermediate distributing frame, and thence to the tip and ring
contacts of the answering jack. A consideration of this connection will
show that the actual electrical connections so far as already described
are exactly those of Figs. 345 and 347, although those figures omitted
the main and intermediate distributing frames. Only two limbs of the
line are involved in the main frame. In the intermediate frame the test
wire running through the multiple is also involved. This test wire, it
will be seen, leads from the test thimbles of all the multiple jacks to
the terminal _3_ on the intermediate frame, thence through the jumper
wire to the terminal _6_ of this frame, and to the test thimble of the
answering jack. Here again the electrical connections are exactly those
represented in Figs. 345 and 347, although those figures do not show the
intermediate frame.
The two terminals _4_ and _5_ of the intermediate frame, besides being
connected to the tip and sleeve springs of the answering jack, are
connected to the contacts of the cut-off relay, and thence through the
coils of the line relay to ground on one side and to battery on the
other. Thus the line relay and battery are normally included in the
circuit of the line. The contact _6_ on the intermediate distributing
frame, besides being connected to the test thimble of all the jacks, is
connected through the coil of the cut-off relay to ground, thus
establishing a path by which current is supplied to the cut-off relay
when connection is made to the line at any jack. There is another
contact _7_ on the intermediate distributing frame which merely forms a
terminal for joining one side of the line lamp to the back contact of
the line relay.
_Functions of Distributing Frames._ Since the line circuit thus far
described in connection with Fig. 348 is exactly the same as that of
Fig. 345 in its electrical connections, it becomes obvious that the main
and intermediate distributing frames play no part in the operation of
the circuit any more than a binding post of a telephone plays a part in
its operation. These frames carry terminals for facilitating the
connection of the various wires in the line circuit and, as will be
shown later, for facilitating certain changes in the line connection.
[Illustration: Fig. 348. Line Circuit No. 1 Board]
Remembering that the dotted lines in Fig. 348 indicate jumper wires of
the main and intermediate distributing frames, and that these are in the
nature of temporary or readily changeable connections, and that the full
lines, whether heavy or light, are permanent connections not readily
changeable, it will be seen that the wires leading through the multiple
jacks of a certain line are permanently associated with each other, and
with certain terminals on the main distributing frame and certain other
terminals on the intermediate distributing frame. It will also be seen
that the line lamp and the answering jack, together with the cut-off
relay and line relay, are permanently associated with each other and
with another group of terminals _4_, _5_, _6_, and _7_ on the
intermediate distributing frame. It will also be apparent that by
changing the jumper wires on the main frame, any outside line may be
connected with any different set of line switchboard equipment, and also
that by making changes in the jumper wires on the intermediate frame,
any given answering jack and line lamp with its associated line cut-off
relay may be associated with any set of multiple jacks.
_Pilot Signals._ In a portion of the circuit leading from the battery
that is common to a group of line lamps is the winding of the pilot
relay, which is common to this group of line lamps. This controls, as
already described, the circuit of the pilot lamp common to the same
group of line lamps. In addition, a night-bell circuit is sometimes
provided, this usually being in the form of an ordinary polarized
ringer, the circuit of which is controlled by a night-bell relay common
to the entire office. Normally, this relay is shunted out of the circuit
of the common portion of the lead to the pilot relay contacts by the key
_8_, but when the key _8_ is opened all current that is fed to the pilot
lamps passes through the night-bell relay, and thus, whenever any pilot
lamp is lighted, the night-bell relay will attract its armature and thus
close the circuit of the calling generator through the night bell.
A study of this figure will make clear to the student how the portions
of the circuit that are individual to the line are associated with such
things as the battery, that are common to the entire office, and such as
the pilot relay and lamp, that are common to a group of lines
terminating in one position.
_Modified Relay Windings._ In some cases, the line relay instead of
being double wound, as shown, is made with a single winding, this
winding being normally included between the ring side of the cut-off
relay and the battery, the tip side of the cut-off relay being run
direct to ground. The present practice of the Western Electric Company
is towards the double-wound relay, however, and that is considered
standard in all of their large No. 1 multiple boards, except where the
customer, owing to special reasons, demands a single wound relay on the
ring side of the line. The prime reason for the two-winding line relay
is the lessened click in the calling-subscriber's receiver which occurs
when the operator answers. All line relays prior to 1902 were
single-wound, but after that they were made double and used some turns
of resistance wire to limit the normal calling current.
_Relay Mounting._ In the standard No. 1 relay board of the Western
Electric Company and, in fact, in nearly all common-battery multiple
boards that are manufactured by other companies, the line and cut-off
relays are mounted on separate racks outside the switchboard room and
adjacent to the main and intermediate distributing frames, the wiring
being extended from the relays to the jacks and lamps on the switchboard
proper by means of suitable cables. The Western Electric Company has
recently instituted a departure from this practice in the case of some
of their smaller No. 1 switchboard installations. Where it is thought
that the ultimate capacity required by the board will not be above 3,000
lines, the relay rack is dispensed with and all of the line and cut-off
relays, as well as the supervisory relays, are mounted in the rear of
the switchboard frame. For this purpose the line and cut-off relays are
specially made with the view to securing the utmost compactness. In
still other cases, in switchboards of relatively small ultimate
capacity, they use this small line and cut-off relay mounted on a
separate relay rack, in which case the board is the standard No. 1 board
except for the type of relays. In all of these modifications of the No.
1 board adapted for the use of the smaller and cheaper relays, the line
relay has but a single winding, the small size of the relay winding not
lending itself readily to double winding with the added necessary coil
terminals.
_Capacity Range._ The No. 1 Western Electric board is made in standard
sizes up to an ultimate capacity of 9,600 lines. For all capacities
above 4,900 lines, a 3/8-inch jack, vertical and horizontal face
dimensions, is employed. For this capacity the smaller types of cut-off
and line relays are not employed. Up to ultimate capacities of 4,900
lines, 1/2-inch jacks are employed, and either the small or the large
relays mounted on a separate rack are available. Up to 3,000 lines
ultimate capacity, the 1/2-inch jack is employed, and either the small
or the large cut-off and line relays are available, but in case the
small type is used the purchaser has the option of mounting them on a
separate relay rack, as in ordinary practice, or mounting them in the
switchboard cabinet and dispensing with the relay rack.
=Western Electric No. 10 Board.= The No. 1 common-battery multiple
switchboard, regardless of its size and type of arrangement of line and
cut-off relays, involves two relays for each line, the line relay
energized by the taking of the receiver off its hook, and the cut-off
relay energized by the act of the operator on plugging in and serving to
remove the line relay from the circuit whenever and as long as a plug is
inserted into any jack of the line. This seems to involve a considerable
expense in relays, but this, as has been stated, is warranted by the
greater simplicity in jacks which the use of the cut-off relay makes
possible. In addition to this expense of investment in the line and
cut-off relays, the amount of current required to hold up the cut-off
relays during conversations foots up to a considerable item of expense,
particularly as the system of supervisory signals is one in which the
supervisory lamp takes current not only while burning, but its circuit
takes even more current when the lamp is extinguished during the time of
a connection. For all of these reasons, and some other minor ones, it
was deemed expedient by the engineers of the Western Electric Company to
design a common-battery multiple switchboard for small and medium-sized
exchanges in which certain sacrifices might be made to the end of
accomplishing certain savings. The result has been a type of
switchboard, designated the No. 10, which may be found in a number of
Bell exchanges, it being considered particularly adaptable to
installations of from 500 to 3,000 lines. Although this board has been
subject to a good deal of adverse criticism, and although it seems
probable that even for the cheaper boards the No. 1 type with some of
the modifications just described will eventually supersede this No. 10
board, yet the present extent of use of the No. 10 board and the
instructive features which its type displays warrant its discussion
here.
_Circuits._ The circuits of this switchboard are shown in Fig. 349, this
indicating two-line circuits and a connecting cord circuit, together
with the auxiliary apparatus employed in connection with the operator's
telephone circuit, the pilot and night alarm circuits. The most
noticeable feature is that cut-off jacks are employed, the circuit of
the line normally extending through the sets of jack springs in the
multiple, and answering jacks to the line relay and battery on one side
of the line, and to ground on the other side. Obviously, the additional
complexity of the jack saves the use of a cut-off relay and the relay
equipment of each line consists, therefore, of but a single line relay,
which controls the lamp in an obvious manner.
[Illustration: Fig. 349. Western Electric No. 10 Board]
The cord circuit is of the three-conductor type, the two talking strands
extending to the usual split repeating-coil arrangement, and battery
current for talking purposes being fed through these windings as in the
standard No. 1 board. The supervisory relay is included in the ring
strand of the cord circuit and is shunted by a non-inductive resistance,
so that its impedance will not interfere with the talking currents. The
armature of the supervisory relay closes the lamp contact on its back
stroke, so that the lamp is always held extinguished when the relay is
energized. The supervisory lamp is included in a connection between the
back contact of the supervisory relay and ground, this connection
including the central-office battery. As a result, the illumination of
the supervisory lamp is impossible until a plug has been inserted into a
jack, in which case, assuming the supervisory relay to be de-energized,
the lamp circuit is completed through the wire connecting all of the
test thimbles and the resistance permanently bridged to ground from that
wire.
_Test._ For purposes of the test it is evident that the test rings of an
idle line are always at ground potential, due to their connection to
ground through the resistance coil. It is also evident that the tip of
an unused calling plug will always be at ground potential and,
therefore, that the testing of an idle line will result in no click in
the operator's receiver. When a line is switched, however, the potential
of all the test rings will be raised due to their being connected with
the live pole of the battery through the third strand of the cord. When
the operator in testing touches the test contact of the jack of a busy
line, a current will, therefore, flow from this test contact to the tip
strand of the cord and thence to ground through one of the repeating
coil windings. The potential of the tip side of the cord will,
therefore, be momentarily altered, and this will result in a click in
the operator's receiver bridged across the cord circuit at the time. The
details of the operator's cord circuit and of the pilot lamp and night
alarm circuits will be clear from the diagram.
_Operation._ A brief summary of the operation of this system is as
follows:
The subscriber removes his receiver from its hook, thus drawing up the
armature of the line relay and lighting his line lamp. The operator
answers. The line lamp is extinguished by the falling back of the
line-relay armature, due to the breaking of the relay circuit at the
jack contacts. The subscriber then receives current for his transmitter
through the cord-circuit battery connections. The supervisory relay
connected with the answering cord is not lighted, because, although the
lamp-circuit connection is completed at the jack, the supervisory relay
is operated to hold the lamp circuit open. Conversation ensues between
the operator and the subscriber, after which the operator tests the line
called for with the tip of the calling plug of the pair used in
answering. If the called line is not busy, no click will ensue, because
both the tested ring and the calling plug are at the same potential.
Finding no click, the operator will insert the plug and ring by means of
the ringing key. When the operator plugs in, the supervisory lamp,
associated with the calling plug, becomes lighted because the circuit is
completed at the jack and the supervisory relay remains de-energized,
since the line circuit is open at the subscriber's station. When the
called subscriber responds, the calling supervisory lamp goes out
because of the energization of the supervisory relay. Both lamps remain
out during the conversation, but when either subscriber hangs up, the
corresponding supervisory lamp will be lighted because of the falling
back of the supervisory relay armature.
If the called line is busy, a click will be heard, for the reason
described, and the operator will so inform the calling subscriber. It
goes without saying, that in any multiple-switchboard system a plug may
be found in the actual multiple jack that is reached for, in which case,
although no test will be made, the busy condition will be reported back
to the calling subscriber.
_Economy._ It has been the belief of the Western Electric engineers that
a real economy is accomplished in this type of board by the saving in
relay equipment. It is, of course, apparent at a glance that with a
switchboard long enough and of sections enough, the cost of extra jack
springs and their platinum contacts must become great enough to offset
the saving accomplished by omitting the cut-off relay. This makes it
apparent that if there is any economy in this type of multiple
switchboard, it must be found in the very small boards where there are
but few jacks per line and where the extra cost of the cut-off jack is
not enough to offset the extra cost of an added relay. It is the growing
belief, however, among engineers, that the multiple switchboard must be
very small indeed in order that the added complexity of the cut-off
jacks and wiring may be able to save anything over the two-relay type of
line; and it is believed that where economy is necessary in small
boards, it may be best effected by employing cheaper and more compact
forms of relays and mounting them, if necessary, directly in the
switchboard cabinet.
NOTE. These two standard types of common-battery multiple
switchboards of the Western Electric Company represent the
development through long years of careful work on the part of
the Western Electric and Bell engineers, credit being
particularly due to Scribner, McBerty, and McQuarrie of the
Western Electric Company, and Hayes of the American Telephone
and Telegraph Company.
=Kellogg Two-Wire Multiple Board.= The simplicity in the jacks permitted
by the use of the cut-off relay in the Western Electric common-battery
multiple switchboard for larger exchanges was carried a step further by
Dunbar and Miller in the development of the so-called two-wire
common-battery multiple switchboard, which for many years has been the
standard of the Kellogg Switchboard and Supply Company. The particular
condition which led to the development of the two-wire system was the
demand at that time on the Kellogg Company for certain very large
multiple switchboards, involving as many as 18,000 lines in the
multiple. Obviously, this necessitated a small jack, and obviously a
jack having only two contacts, a tip spring and a sleeve, could be made
more easily and with greater durability of this very small size than a
jack requiring three or more contacts. Other reasons that were
considered were, of course, cheapness in cost of construction and
extreme simplicity, which, other things being equal, lends itself to low
cost of maintenance.
_Line Circuit._ Like the standard Western Electric board for large
offices, the Kellogg two-wire board employs two relays for each line,
the line relay under the control of the subscriber and in turn
controlling the lamp, and a cut-off relay under the control of the
operator and in turn controlling the connection of the line relay with
the line. The line circuit as originally developed and as widely used by
the Kellogg Company is shown in Fig. 350. The extreme simplicity of the
jacks is apparent, as is also the fact that but two wires lead through
the multiple. Another distinguishing feature is, that all of the
multiple and answering jacks are normally cut off from the line at the
cut-off relay, but when the cut-off relay operates it serves, in
addition to cutting off the line relay, to attach the two limbs of the
line to the two wires leading through the multiple and answering jacks.
The control of the line relay by the subscriber's switch hook is clear
from the figure. The control of the cut-off relay is secured by
attaching one terminal of the cut-off relay winding permanently to that
wire leading through the multiple which connects with the sleeve
contacts of the jack, the other terminal of the cut-off relay being
grounded. The way in which this relay is operated will be understood
when it is stated that the sleeve contacts of both the answering and
calling plugs always carry full battery potential and, therefore,
whenever any plug is inserted into any jack, current flows from the
sleeve of the jack through the sleeve contact of the jack to ground,
through the winding of the cut-off relay, which relay becomes energized
and performs the functions just stated. It is seen that the wire
running through the multiple to which the sleeve jack contacts are
attached, is thus made to serve the double purpose of answering as one
side of the talking circuit, and also of performing the functions
carried out by the separate or third wire in the three-wire system. It
will be shown also that, in addition, this wire is made to lend itself
to the purposes of the busy test without any of these functions
interfering with each other in any way.
[Illustration: Fig. 350. Two-Wire Line Circuit]
_Cord Circuit._ The cord circuit in somewhat simplified form is shown in
Fig. 351. Here again there are but two conductors to the plugs and two
strands to the cords. This greater simplicity is in some measure offset
by the fact that four relays are required, two for each plug. This
so-called four-relay cord circuit may be most readily understood by
considering half of it at a time, since the two relays associated with
the answering plug act in exactly the same way as those connected with
the calling plug.
[Illustration: Fig. 351. Two-Wire Cord Circuit]
Associated with each plug of a pair are two relays _1_ and _2_, in the
case of the answering cord, and _3_ and _4_ in the case of the calling
cord. The coils of the relays _1_ and _2_ are connected in series and
bridged across the answering cord, a battery being included between the
coils in this circuit. The coils of the relays _3_ and _4_ are similarly
connected across the calling cord. A peculiar feature of the Kellogg
system is that two batteries are used in connection with the cord
circuit, one of them being common to all answering cords and the other
to all calling cords. The operation of the system would, however, be
exactly the same if a single battery were substituted for the two.
_Supervisory Signals._ Considering the relays associated with the
answering cord, it is obvious that these two relays _1_ and _2_ together
control the circuit of the supervisory lamp _5_, the circuit of this
lamp being closed only when the relay _1_ is de-energized and the relay
_2_ is energized. We will find in discussing the operation of these that
the relay _2_ is wholly under the control of the operator, and that the
relay _1_, after its plug has been connected with a line, is wholly
under the control of the subscriber on that line. It is through the
windings of these two relays that current is fed to the line of the
subscriber connected with the corresponding cord.
When a plug--the answering plug, for instance--is inserted into a jack,
current at once flows from the positive pole of the left-hand battery
through the winding of the relay _2_ to the sleeve of the plug, thence
to the sleeve of the jack and through the cut-off relay to ground. This
at once energizes the supervisory relay _2_ and the cut-off relay
associated with the line. The cut-off relay acts, as stated, to continue
the tip and sleeve wires associated with the jacks to the line leading
to the subscriber, and also to cut off the line relay. The supervisory
relay _2_ acts at the same time to attract its armature and thus
complete its part in closing the circuit of the supervisory lamp.
Whether or not the lamp will be lighted at this time depends on whether
the relay _1_ is energized or not, and this, it will be seen, depends on
whether the subscriber's receiver is off or on its hook. If off its
hook, current will flow through the metallic circuit of the line for
energizing the subscriber's transmitter, and as whatever current goes to
the subscriber's line must flow through the relay _1_, that relay will
be energized and prevent the lighting of the supervisory lamp _5_. If,
on the other hand, the subscriber's receiver is on its hook, no current
will flow through the line, the supervisory relay will not be energized,
and the lamp _5_ will be lighted.
In a nutshell, the sleeve supervisory relay normally prevents the
lighting of the corresponding supervisory lamp, but as soon as the
operator inserts a plug into the jack of the line, the relay _2_
establishes such a condition as to make possible the lighting of the
supervisory lamp, and the lighting of this lamp is then controlled
entirely by the relay _1_, which is, in turn, controlled by the position
of the subscriber's switch hook.
_Battery Feed._ A 2-microfarad condenser is included in each strand of
the cord, and battery is fed through the relay windings to the calling
and called subscribers on opposite sides of these condensers, in
accordance with the combined impedance coil and condenser method
described in Chapter XIII. Here the relay windings do double duty,
serving as magnets for operating the relays and as retardation coils in
the system of battery supply.
_Complete Cord and Line Circuits._ The complete cord and line circuits
of the Kellogg two-wire system are shown in Fig. 352. In the more recent
installations of the Kellogg Company the cord and line circuits have
been slightly changed from those shown in Figs. 350 and 351, and these
changes have been incorporated in Fig. 352. The principles of operation
described in connection with the simplified figures remain, however,
exactly the same. One of the changes is, that the tip side of the lines
is permanently connected to the tips of the jacks instead of being
normally cut off by the cut-off relay, as was done in the system as
originally developed. Another change is, that the line relay is
associated with the tip side of the line, rather than with the sleeve
side, as was formerly done. The cord circuit shown in Fig. 352 shows
exactly the same arrangement of supervisory relays and exactly the same
method of battery feed as in the simplified cord circuit of Fig. 351,
but in addition to this the detailed connections of the operator's
talking set and of her order-wire keys are indicated, and also the
ringing equipment is indicated as being adapted for four-party harmonic
work.
[Illustration: Fig. 352. Kellogg Two-Wire Board]
In connection with this ringing key it may be stated that the springs
_7_, _8_, _9_, and _10_ are individually operated by the pressure of one
of the ringing key buttons, while the spring _17_, connected with the
sleeve side of the calling plug, is always operated simultaneously with
the operation of any one of the other springs. As a result the proper
ringing circuit is established, it being understood that the upper
contacts of the springs _7_, _8_, _9_, and _10_ lead to the terminals of
their respective ringing generators, the other terminals of which are
grounded. The circuit is, therefore, from the generator, through the
ringing key, out through the tip side of the line, back over the sleeve
side of the line, and to ground through the spring _17_, resistance
_11_, and the battery, which is one of the cord-circuit batteries. The
object of this coil _11_ and the battery connection through it to the
ringing-key spring is to prevent the falling back of the cut-off relay
when the ringing key is operated. This will be clear when it is
remembered that the cut-off relay is energized by battery current fed
over the sleeve strand of the cord, and obviously, since it is necessary
when the ringing key is operated to cut off the supply wire back of the
key, this would de-energize the cut-off relay when the ringing key was
depressed, and the falling back of the cut-off relay contacts would make
it impossible to ring because the sleeve side of the line would be cut
off. The battery supply through the resistance _11_ is, therefore,
substituted on the sleeve strand of the cord for the battery supply
through the normal connection.
_Busy Test._ The busy test depends on all of the test rings being at
zero potential on an idle line and at a higher potential on a busy line.
Obviously, when the line is not switched, the test rings are at zero
potential on account of a ground through the cut-off relay. When,
however, a plug is inserted in either the answering or multiple jacks,
the test rings will all be raised in potential due to being connected
with the live side of the battery through the sleeve strand of the cord.
Conditions on the line external to the central office cannot make an
idle line test busy because, owing to the presence of the cut-off relay,
the sleeve contacts of all the jacks are disconnected from the line when
it is idle. The test circuit from the tip of the calling plug to ground
at the operator's set passes through the tip strand of the cord, thence
through a pair of normally closed extra contacts on the supervisory
relay _4_, thence in series through all the ringing key springs _10_,
_9_, _8_, and _7_, thence through an extra pair of springs _12_ and _13_
on the listening key--closed only when the listening key is
operated--and thence to ground through a retardation coil _14_. No
battery or other source of potential exists in this circuit between
ground and the tip of the calling plug and, therefore, the tip is
normally at ground potential. The sleeve ring of the jack being at
ground potential if the line is idle, no current will flow and no click
will be produced in testing such a line. If, however, the line is busy,
the test ring will be at a higher potential and, therefore, current will
flow from the tip of the calling plug to ground over the path just
traced, and this will cause a rise in potential at the terminal of the
condenser _15_ and a momentary flow of current through the tertiary
winding _16_ of the operator's induction coil; hence the click.
[Illustration: SWITCH ROOM OF CITIZENS' TELEPHONE COMPANY, GRAND RAPIDS,
MICH. One of the Earliest Large Automatic Offices.]
Obviously the testing circuit from the tip of the calling plug to ground
at the operator's set is only useful during the time when the calling
plug is not in a jack, and as the tip strand of the calling plug has to
do double duty in testing and in serving as a part of the talking
circuit, the arrangement is made that the testing circuit will be
automatically broken and the talking circuit through the tip strand
automatically completed when the plug is inserted into a jack in
establishing a connection. This is accomplished by means of the extra
contact on the relay _4_, which relay, it will be remembered, is held
energized when its corresponding plug is inserted in a jack. During the
time when the plug is not inserted, this relay is not energized and the
test circuit is completed through the back contact of its right-hand
armature. When connection is made at the jack, this relay becomes
energized and the tip strand of the cord circuit is made complete by the
right-hand lever being pulled against the front contact of this relay.
The keys shown to the right of the operator's set are order-wire keys.
_Summary of Operation._ We may give a brief summary of the operation of
this system as shown in Fig. 352. The left-hand station calls and the
line relay pulls up, lighting the lamp. The operator inserts an
answering plug in the answering jack, thus energizing the cut-off relay
which operates to cut off the line relay and to complete the connection
between the jacks and the external line. The act of plugging in by the
operator also raises the potential of all the test rings so as to guard
the line against intrusion by other callers. The supervisory lamp _5_
remains unlighted because, although the relay _2_ is operated, the relay
_1_ is also operated, due to the calling subscriber's receiver being off
its hook. The operator throws her listening key, communicates with the
subscriber, and, learning that the right-hand station is wanted,
proceeds to test that line. If the line is idle, she will get no click,
because the tip of her calling plug and the tested ring will be at the
same ground potential. She then plugs in and presses the proper
ringing-key button to send out the proper frequency to ring the
particular subscriber on the line--if there be more than one--the
current from the battery through the coil _11_ and spring _17_ serving
during this operation to hold up the cut-off relay.
As soon as the operator plugs in with the calling plug, the supervisory
lamp _6_ lights, assuming that the called subscriber had not already
removed his receiver from its hook, due to the fact that the relay _4_
is energized and the relay _3_ is not. As soon as the called subscriber
responds, the relay _3_ becomes energized and the supervisory lamp goes
out. If the line called for had been busy by virtue of being plugged at
another section, the tip of the operator's plug in testing would have
found the test ring raised to a potential above the ground, and, as a
consequence, current would have flowed from the tip of this plug through
the back contact of the right-hand lever of relay _4_, thence through
the ringing key springs and the auxiliary listening-key springs to
ground through the retardation coil _14_. This would have produced a
click by causing a momentary flow of current through the tertiary
winding _16_ of the operator's set.
_Wiring of Line Circuit._ The more complete wiring diagram of a single
subscriber's line, Fig. 353, shows the placing in the circuits of the
terminals and jumper wires of the main distributing frame and of the
intermediate distributing frame, and also shows how the pilot lamps and
night-alarm circuits are associated with a group of lines. The main
distributing frame occupies the same relative position in this line
circuit as in the Western Electric, being located in the main line
circuit outside of all the switchboard apparatus. The intermediate
distributing frame occupies a different relative position from that in
the Western Electric line. It will be recalled by reference to Fig. 348
that the line lamp and the answering jack were permanently associated
with the line and cut-off relays, such mutations of arrangement as were
possible at the intermediate distributing frame serving only to vary the
connection between the multiple of a line and one of the various groups
of apparatus consisting of an answering jack and line lamp and
associated relays. In the Kellogg arrangement, Fig. 353, the line and
cut-off relays, instead of being permanently associated with the
answering jack and line lamp, are permanently associated with the
multiple jacks, no changes, of which the intermediate or main frames are
capable, being able to alter the relation between a group of multiple
jacks and its associated line and cut-off relays. In this Kellogg
arrangement the intermediate distributing frame may only alter the
connection of an answering jack and line lamp with the multiple and its
permanently associated relays. The pilot and night alarm arrangements of
Fig. 353 should be obvious from the description already given of other
similar systems.
[Illustration: Fig. 353. Kellogg Two-Wire Line Circuit]
=Dean Multiple Board.= In Fig. 354 are shown the circuits of the
multiple switchboard of the Dean Electric Company. The subscriber's
station equipment shown at Station _A_ and Station _B_ will be
recognized as the Wheatstone-bridge circuit of the Dean Company.
_Line Circuit._ The line circuit is easily understood in view of what
has been said concerning the Western Electric line circuit, the line
relay _1_ being single wound and between the live side of the battery
and the ring side of the line. The cut-off relay _2_ is operated
whenever a plug is inserted in a jack and serves to sever the connection
of the line with the normal line signaling apparatus.
_Cord Circuit._ The cord circuit is of the four-relay type, but employs
three conductors instead of two, as in the two-wire system. The relay
_3_, being in series between the battery and the sleeve contact on the
plug, is energized whenever a plug is inserted in the jack, its winding
being placed in series with the cut-off relay of the line with which the
plug is connected. This completes the circuit through the associated
supervisory lamp unless the relay _4_ is energized, the local lamp
circuit being controlled by the back contact of relay _4_ and the front
contact of relay _3_. It is through the two windings of the relay _4_
that current is fed to the subscriber's station, and, therefore, the
armature of this relay is responsive to the movements of the
subscriber's hook. As the relay _3_ holds the supervisory lamp circuit
closed as long as a plug is inserted in a jack of the line, it follows
that during a connection the relay _4_ will have entire control of the
supervisory lamp.
_Listening Key._ The listening key, as usual, serves to connect the
operator's set across the talking strands of the cord circuit, and the
action of this in connection with the operator's set needs no further
explanation.
_Ringing Keys._ The ringing-key arrangement illustrated is adapted for
use with harmonic ringing, the single springs _5_, _6_, _7_, and _8_
each being controlled by a separate button and serving to select the
particular frequency that is to be sent to line. The two springs _9_ and
_10_ always act to open the cord circuit back of the ringing keys,
whenever any one of the selective buttons is depressed, in order to
prevent interference by ringing current with the other operations of the
circuit.
Two views of these ringing keys are shown in Figs. 355 and 356. Fig. 356
is an end view of the entire set. In Fig. 355 the listening key is shown
at the extreme right and the four selective buttons at the left. When a
button is released it rises far enough to cause the disengagement of the
contacts, but remains partially depressed to serve as an indication that
it was last used. The group of springs at the extreme left of Fig. 355
are the ones represented at _9_ and _10_ in Fig. 354 and by the anvils
with which those springs co-operate.
[Illustration: Fig. 354. Dean Multiple Board Circuits]
_Test._ The test in this Dean system is simple, and, like the Western
Electric and Kellogg systems, it depends on the raising of the
potential of the test thimbles of all the line jacks of a line when a
connection is made with that line by a plug at any position. When an
operator makes a test by applying the tip of the calling plug to the
test thimble of a busy line, current passes from the test thimble
through the tip strand of the cord to ground through the left-hand
winding of the calling supervisory relay _4_. The drop of potential
through this winding causes the tip strand of the cord to be raised to a
higher potential than it was before, and as a result the upper plate of
the condenser _11_ is thus altered in potential and this change in
potential across the condenser results in a click in the operator's ear.
[Illustration: Fig. 355. Dean Party Line Ringing Key]
[Illustration: Fig. 356. Dean Party Line Ringing Key]
=Stromberg-Carlson Multiple Board.= _Line Circuit._ In Fig. 357 is shown
the multiple common-battery switchboard circuits employed by the
Stromberg-Carlson Telephone Manufacturing Company. The subscriber's line
circuits shown in this drawing are of the three-wire type and, with the
exception of the subscriber's station, are the same as already described
for the Western Electric Company's system.
_Cord Circuit._ The cord circuit employed is of the two-conductor type,
the plugs being so constructed as to connect the ring and thimble
contacts of the jack when inserted. This cord circuit is somewhat
similar to that employed by the Kellogg Switchboard and Supply Company,
shown in Fig. 352, except that only one battery is employed, and that
certain functions of this circuit are performed mechanically by the
inter-action of the armatures of the relays.
_Supervisory Signals._ When the answering plug is inserted in a jack, in
response to a call, the current passing to the subscriber's station and
also through the cut-off relay must flow through the relay _1_, thus
energizing it. As the calling subscriber's receiver is at this time
removed from the hook switch, the path for current will be completed
through the tip of the jack, thence through the tip of the plug, through
relay _2_ to ground, causing relay _2_ to be operated and to break the
circuit of the answering supervisory lamp. The two relays _1_ and _2_
are so associated mechanically that the armature of _1_ controls the
armature of _2_ in such a manner as to normally hold the circuit of the
answering supervisory lamp open. But, however, when the plug is inserted
in a jack, relay _1_ is operated and allows the operation of relay _2_
to be controlled by the hook switch at the subscriber's station. The
supervisory relay _3_ associated with the calling cord is operated when
the calling plug is placed in a jack, and this relay normally holds the
armature of relay _4_ in an operated position in a similar manner as the
armature of relay _1_ controlled that of relay _2_. Supervisory relay
_4_ is under the control of the hook switch at the called subscriber's
station.
_Test._ In this circuit, as in several previously described, when a plug
is inserted in a jack of a line, the thimble contacts of the jacks
associated with that line are raised to a higher potential than that
which they normally have. The operator in testing a busy line, of course
having previously moved the listening key to the listening position,
closes a path from the test thimble of the jack, through the tip of the
calling plug, through the contacts of the relay _4_, the inside springs
of the listening key, thence through a winding of the induction coil
associated with her set to ground. The circuit thus established allows
current to flow from the test thimble of the jack through the winding of
her induction coil to ground, causing a click in her telephone receiver.
The arrangement of the ringing circuit does not differ materially from
that already described for other systems and, therefore, needs no
further explanation.
[Illustration: Fig. 357. Stromberg-Carlson Multiple Board Circuits]
=Multiple Switchboard Apparatus.= Coming now to a discussion of the
details of apparatus employed in multiple switchboards, it may be
stated that much of the apparatus used in the simpler types is capable
of doing duty in multiple switchboards, although, of course,
modification in detail is often necessary to make the apparatus fit the
particular demands of the system in which it is to be used.
_Jacks._ Probably the most important piece of apparatus in the multiple
switchboard is the jack, its importance being increased by the fact that
such very large numbers of them are sometimes necessary. Switchboards
having hundreds of thousands of jacks are not uncommon. The multiple
jacks are nearly always mounted in strips of twenty and the answering
jacks usually in strips of ten, the length of the jack strip being the
same in each case in the same board and, therefore, giving twice as wide
a spacing in the answering as in the multiple jacks. The distance
between centers in the multiple jacks varies from a quarter of an
inch--which is perhaps the extreme minimum--to half an inch, beyond
which larger limit there seems to be no need of going in any case. It is
customary that the jack strip shall be made of the same total thickness
as the distance between the centers of two of its jacks, and from this
it follows that the strips when piled one upon the other give the same
vertical distance between jack centers as the horizontal distance.
In Fig. 358 is shown a strip of multiple and a strip of answering jacks
of Western Electric make, this being the type employed in the No. 1
standard switchboards for large exchanges. In Fig. 359 are shown the
multiple and answering jacks employed in the No. 10 Western Electric
switchboard. The multiple jacks in the No. 1 switchboard are mounted on
3/8-inch centers, the jacks having three branch terminal contacts. The
multiple jacks of the No. 10 switchboard indicated in Fig. 359 are
mounted on 1/2-inch centers, each jack having five contacts as indicated
by the requirement of the circuits in Fig. 349.
In Fig. 360 are shown the answering and multiple jacks of the Kellogg
Switchboard and Supply Company's two-wire system. The extreme simplicity
of these is particularly well shown in the cut of the answering jack,
and these figures also show clearly the customary method of numbering
jacks. In very large multiple boards it has been the practice of the
Kellogg Company to space the multiple jacks on 3/10-inch centers, and in
their smaller multiple work, they employ the 1/2-inch spacing. With the
3/10-inch spacing that company has been able to build boards having a
capacity of 18,000 lines, that many jacks being placed within the reach
of each operator.
In all modern multiple switchboards the test thimble or sleeve contacts
are drawn up from sheet brass or German silver into tubular form and
inserted in properly spaced borings in strips of hard rubber forming the
faces of the jacks. These strips sometimes are reinforced by brass
strips on their under sides. The springs forming the other terminals of
the jack are mounted in milled slots in another strip of hard rubber
mounted in the rear of and parallel to the front strip and rigidly
attached thereto by a suitable metal framework. In this way desired
rigidity and high insulation between the various parts is secured.
[Illustration: Fig. 358. Answering and Multiple Jacks for No. 1 Board]
_Lamp Jacks._ The lamp jacks employed in multiple work need no further
description in view of what has been said in connection with lamp jacks
for simple common-battery boards. The lamp jack spacing is always the
same as the answering jack spacing, so that the lamps will come in the
same vertical alignment as their corresponding answering jacks when the
lamp strips and answering jack strips are mounted in alternate layers.
[Illustration: Fig. 359. Answering and Multiple Jacks for No. 10 Board]
[Illustration: Fig. 360. Answering and Multiple Jacks for Kellogg
Two-Wire Board]
_Relays._ Next in order of importance in the matter of individual parts
for multiple switchboards is the relay. The necessity for reliability of
action in these is apparent, and this means that they must not only be
well constructed, but that they must be protected from dust and moisture
and must have contact points of such a nature as not to corrode even in
the presence of considerable sparking and of the most adverse
atmospheric conditions. Economy of space is also a factor and has led to
the almost universal adoption of the single-magnet type of relay for
line and cut-off as well as supervisory purposes.
[Illustration: Fig. 361. Type of Line Relay]
[Illustration: Fig. 362. Type of Cut-Off Relay]
The Western Electric Company employs different types of relays for line,
cut-off, and supervisory purposes. This is contrary to the practice of
most of the other companies who make the same general type of relay
serve for all of these purposes. A good idea of the type of Western
Electric line relay, as employed in its No. 1 board, may be had from
Fig. 361. As is seen this is of the tilting armature type, the armature
rocking back and forth on a knife-edge contact at its base, the part on
which it rests being of iron and of such form as to practically
complete, with the armature and core, the magnetic circuit. The cut-off
relay, Fig. 362, is of an entirely different type. The armature in this
is loosely suspended by means of a flexible spring underneath two
L-shaped polar extensions, one extending up from the rear end of the
core and the other from the front end. When energized this armature is
pulled away from the core by these L-shaped pieces and imparts its
motion through a hard-rubber pin to the upper pair of springs so as to
effect the necessary changes in the circuit.
[Illustration: Fig. 363. Western Electric Combined Line and Cut-off
Relay]
[Illustration: Fig. 364. Western Electric Supervisory Relay]
[Illustration: Fig. 365. Line Relay No. 10 Board]
Much economy in space and in wiring is secured in the type of
switchboards employing cut-off as well as line relays by mounting the
two relays together and in making of them, in fact, a unitary piece of
apparatus. Since the line relay is always associated with the cut-off
relay of the same line and with no other, it is obvious that this
unitary arrangement effects a great saving in wiring and also secures a
great advantage in the matter of convenience of inspection. Such a
combined cut-off and line relay, employed in the Western Electric No. 1
relay board, is shown in Fig. 363. These are mounted in banks of ten
pairs, a common dust cap of sheet iron covering the entire group.
The Western Electric supervisory relay, Fig. 364, is of the tilting
armature type and is copper clad. The dust cap in this case fits on with
a bayonet joint as clearly indicated. In Fig. 365 is shown the line
relay employed in the Western Electric No. 10 board.
[Illustration: Fig. 366. Kellogg Line and Cut-off Relays]
[Illustration: Fig. 367. Strip of Kellogg Line and Cut-Off Relays]
The Kellogg Company employs the type of relay of which the magnetic
circuit was illustrated in Fig. 95. In its multiple boards it commonly
mounts the line and cut-off relays together, as shown in Fig. 366. A
single, soft iron shell is used to cover both of these, thus serving as
a dust shield and also as a magnetic shield to prevent cross-talk
between adjacent relays--an important feature, since it will be
remembered the cut-off relays are left permanently connected with the
talking circuit. Fig. 367, which shows a strip of twenty such pairs of
relays, from five of which the covers have been removed, is an excellent
detail view of the general practice in this respect; obviously, a very
large number of such relays may be mounted in a comparatively small
space. The mounting strip shown in this cut is of heavy rolled iron and
is provided with openings through which the connection terminals--shown
more clearly in Fig. 366--project. On the back of this mounting strip
all the wiring is done and much of this wiring--that connecting adjacent
terminals on the back of the relay strip--is made by means of thin
copper wires without insulation, the wires being so short as to support
themselves without danger of crossing with other wires. When these wires
are adjacent to ground or battery wires they may be protected by
sleeving, so as to prevent crosses.
[Illustration: Fig. 368. Monarch Relay]
An interesting feature in relay construction is found in the relay of
the Monarch Telephone Manufacturing Company shown in Figs. 368 and 369.
The assembled relay and its mounting strip and cap are shown in Fig.
368. This relay is so constructed that by the lifting of a single latch
not only the armature but the coil may be bodily removed, as shown in
Fig. 369, in which the latch is shown in its raised position. As seen,
the armature has an L-shaped projection which serves to operate the
contact springs lying on the iron plate above the coil. The simplicity
of this device is attractive, and it is of convenience not only from the
standpoint of easy repairs but also from the standpoint of factory
assembly, since by manufacturing standard coils with different
characters of windings and standard groups of springs, it is possible to
produce without special manufacture almost any combination of relay.
[Illustration: Fig. 369. Monarch Relay]
=Assembly.= The arrangement of the key and jack equipment in complete
multiple switchboard sections is clearly shown in Fig. 370, which shows
a single three-position section of one of the small multiple
switchboards of the Kellogg Switchboard and Supply Company. The
arrangement of keys and plugs on the key shelf is substantially the same
as in simple common-battery boards. As in the simple switchboards the
supervisory lamps are usually mounted on the hinged key shelf
immediately in the rear of the listening and ringing keys and with such
spacing as to lie immediately in front of the plugs to which they
correspond. The reason for mounting the supervisory lamps on the key
shelf is to make them easy of access in case of the necessity of lamp
renewals or repairs on the wiring. The space at the bottom of the
vertical panels, containing the jacks, is left blank, as this space is
obstructed by the standing plugs in front of it. Above the plugs,
however, are seen the alternate strips of line lamps and answering
jacks, the lamps in each case being directly below the corresponding
answering jacks. Above the line lamps and answering jacks in the two
positions at the right there are blank strips into which additional line
lamps and jacks may be placed in case the future needs of the system
demand it. The space above these is the multiple jack space, and it is
evident from the small number of multiple jacks in this little
switchboard that the present equipment of the board is small. It is also
evident from the amount of blank space left for future installations of
multiple jacks that a considerable growth is expected. Thus, while there
are but four banks of 100 multiple jacks, or 400 in all, there is room
in the multiple for 300 banks of 100 multiple jacks, or 3,000 in all.
The method of grouping the jacks in banks of 100 and of providing for
their future growth is clearly indicated in this figure. The next
section at the right of the one shown would contain a duplicate set of
multiple jacks and also an additional equipment of answering jacks and
lamps.
[Illustration: A MULTIPLE MANUAL SWITCHING BOARD FOR TOLL CONNECTIONS IN
AN AUTOMATIC SYSTEM Multiple Jacks are Provided for Each Line through
Which Toll Connections are Handled Directly.]
[Illustration: Fig. 370. Small Multiple Board Section]
For ordinary local service no operator would sit at the left-hand
position of the section shown, that being the end position, since the
operator there would not be able easily to reach the extreme right-hand
portion of the third position and would have nothing to reach at her
left. This end position in this particular board illustrated is provided
with toll-line equipment, a practice not uncommon in small multiple
boards. To prevent confusion let us assume that the multiple jack space
contains its full equipment of 3,000 jacks on each section. The
operator in the center position of the section shown could easily reach
any one of the jacks on that section. The operator at the third position
could reach any jack on the second and third position of her section,
but could not well reach multiple jacks in the first position. She
would, however, have a duplicate of the multiple jacks in this first
position in the section at her right, _i. e._, in the fourth position,
and it makes no difference on what portion of the switchboard she plugs
into the multiple so long as she plugs into a jack of the right line.