TRANSFER SWITCHBOARD
When the traffic originating in a switchboard becomes so great as to
require so many operators that the board must be made so long that any
one of the operators cannot reach over its entire face, the simple
switchboard does not suffice. Either some form of transfer switchboard
or of multiple switchboard must be used. In this chapter the transfer
switchboard will be briefly discussed.
The transfer switchboard is so named because its arrangement is such
that some of the connections through it are handled by means of two
operators, the operator who answers the call transferring it to another
operator who completes the connection desired.
=Limitations of Simple Switchboard.= Conceive a number of simple magneto
switchboards, or a number of common-battery switchboards, arranged side
by side, their number being so great as to form, by their combination, a
board too long for the ordinary cords and plugs to reach between its
extremities. On each of these simple switchboards, which we will say are
each of the one-position type, there terminates a group of subscribers'
lines so great in number, considering the traffic on them, that the
efforts of one operator will just about be taxed to properly attend to
their calls during the busiest hours of the day. If, now, these
subscribers would be sufficiently accommodating to call for no other
subscribers than those whose lines terminate on the same switchboard
section or on one of the immediately adjacent switchboard sections, all
would be well, but subscribers will not be so restricted. They demand
universal service; that is, they demand the privilege of having their
own lines connected with the line of any other person in the exchange.
Obviously, in the arrangement just conceived, any operator may answer
any call originating at her own board and complete the connection with
the desired subscriber if that subscriber's jack terminates on her own
section or on one of the adjacent ones. Beyond that she is powerless
unless other means are provided.
=Transfer Lines.= In the transfer board these other means consist in the
provision of groups of local trunk lines or transfer lines extending
from each switchboard position to each other non-adjacent switchboard
position. When an operator receives a call for some line on a
non-adjacent position, having answered this call with her answering
plug, she inserts the calling plug into the jack of one of these
transfer lines that leads to the proper other section. The operator at
that section is notified either verbally or by signal, and she completes
the connection between the other end of the transfer line and the line
of the called subscriber; the connection between the two subscribers
thus being effected through the cords of the two operators in question
linked together by the transfer line. Such a transfer line as just
described, requiring the connection at each of its ends by one of the
plugs of the operator's cord pair, is termed a _jack-ended trunk_ or a
_jack-ended transfer line_ because each of its ends terminates in a
jack.
[Illustration: Fig. 330. Jack-Ended Transfer Circuit]
There is another method of accomplishing the same general result by the
employment of the so-called _plug-ended trunk_ or _plug-ended transfer
line_. In this the trunk or transfer line terminates at one end, the
answering end, in a jack as before, and the connection is made with it
by the answering operator by means of the calling plug of the pair with
which she answered the originating call. The other end of this trunk,
instead of terminating in a jack, ends in a plug and the second operator
involved in the connection, after being notified, picks up this plug and
inserts it in the jack of the called subscriber, thus completing the
connection without employing one of her regular cord pairs.
_Jack-Ended Trunk._ In Fig. 330 are shown the circuits of a commonly
employed jack-ended trunk for transfer boards. The talking circuit, as
usual, is shown in heavy lines and terminates in the tip and sleeve of
the transfer jacks at each end. The auxiliary contacts in these jacks
and the circuits connecting them are absolutely independent of the
talking circuit and are for the purpose of signaling only, the
arrangement of the jacks being such that when a plug is inserted, the
spring _1_ will break from spring _2_ and make with spring _3_.
Obviously, the insertion of a plug in either of the jacks will establish
such connections as to light both lamps, since the engagement of spring
_1_ with spring _3_ in either of the jacks will connect both of the
lamps in multiple across the battery, this connection including always
the contacts _1_ and _2_ of the other jack. From this it follows that
the insertion of a plug in the other end of the trunk will, by breaking
contact between springs _1_ and _2_, put out both the lamps. One plug
inserted will, therefore, light both lamps; two plugs inserted or two
plugs withdrawn will extinguish both lamps.
[Illustration: Fig. 331. Jack-Ended Transfer Circuit]
If an operator located at one end of this trunk answers a call and finds
that the called-for subscriber's line terminates within reach of the
operator near the other end of this trunk, she will insert a calling
plug, corresponding to the answering plug used in answering a call, into
the jack of this trunk and thus light the lamp at both its ends. The
operator at the other end upon seeing this transfer lamp illuminated
inserts one of her answering plugs into the jack, and by means of her
listening key ascertains the number of the subscriber desired, and
immediately inserts her calling plug into the jack of the subscriber
wanted and rings him in the usual manner. The act of this second
operator in inserting her answering plug into the jack extinguishes the
lamp at her own end and also at the end where the call originated, thus
notifying the answering operator that the call has been attended to. As
long as the lamps remain lighted, the operators know that there is an
unattended connection on that transfer line. Such a transfer line is
called a _two-way_ line or a _single-track_ line, because traffic over
it may be in either direction. In Fig. 331 is shown a trunk that
operates in a similar way except that the two lamps, instead of being
arranged in multiple, are arranged in series.
[Illustration: Fig. 332. Jack- and Plug-Ended Transfer Circuit]
_Plug-Ended Trunk._ In Fig. 332 is shown a plug-ended trunk, this
particular arrangement of circuits being employed by the Monarch Company
in its transfer boards. This is essentially a one-way trunk, and traffic
over it can pass only in the direction of the arrow. Traffic in the
opposite direction between any two operators is handled by another trunk
or group of trunks similar to this but "pointed" in the other direction.
For this reason such a system is referred to as a _double-track_ system.
The operation of signals is the same in this case as in Fig. 330, except
that the switching device at the left-hand end of the trunk instead of
being associated with the jack is associated with the plug seat, which
is a switch closely associated with the seat of a plug so as to be
operated whenever the plug is withdrawn from or replaced in its seat.
The operation of this arrangement is as follows: Whenever an operator at
the right-hand end of this trunk receives a call for a subscriber whose
line terminates within the reach of the operator at the left-hand end of
the trunk, she inserts the calling plug of the pair used in answering
the calling subscriber into the jack of the trunk, and thus lights both
of the trunk lamps. The operator at the other end of the trunk, seeing
the trunk lamp lighted, raises the plug from its seat and, having
learned the wishes of the calling subscriber, inserts this plug into the
jack of the called subscriber without using one of her regular pairs.
When she raised the trunk plug from its seat, she permitted the long
spring _1_ of the plug seat switch to rise, thus extinguishing both
lamps and giving the signal to the originating operator that the trunk
connection has received attention. On taking down the connection, the
withdrawal of the plug from the right hand of the trunk lights both
lamps, and the restoring of the trunk plug to its normal seat again
extinguishes both lamps.
=Plug-Seat Switch.= The plug-seat switch is a device that has received a
good deal of attention not only for use with transfer systems, but also
for use in a great variety of ways with other kinds of manual switching
systems. The placing of a plug in its seat or withdrawing it therefrom
offers a ready means of accomplishing some switching or signaling
operation automatically. The plug-seat switch has, however, in spite of
its possibilities, never come into wide use, and so far as we are aware
the Monarch Telephone Manufacturing Company is the only company of
prominence which incorporates it in its regular output. The Monarch
plug-switch mechanism is shown in Fig. 333, and its operation is
obvious. It may be stated at this point that one of the reasons why the
plug-seat switch has not been more widely adopted for use, is the
difficulty that has been experienced due to lint from the switchboard
cords collecting on or about the contact points. In the construction
given in the detailed cut, upper part, Fig. 333, is shown the means
adopted by the Monarch Company for obviating this difficulty. The
contact points are carried in the upper portion of an inverted cup
mounted on the under side of the switchboard shelf, and are thus
protected, in large measure, from the damaging influence of dust and
lint.
[Illustration: Fig. 333. Plug-Seat Switch]
[Illustration: Fig. 334. Order-Wire Arrangement]
=Methods of Handling Transfers.= One way of giving the number of the
called subscriber to the second operator in a transfer system is to
have that operator listen in on the circuit after it is continued to her
position and receive the number either from the first operator or from
the subscriber. Receiving it from the first operator has the
disadvantage of compelling the first operator to wait on the circuit
until the second operator responds; receiving it from the subscriber has
the disadvantage of sometimes being annoying to him. This, however, is
to be preferred to the loss of time on the part of the originating
operator that is entailed by the first method. A better way than either
of these is to provide between the various operators working in a
transfer system, a so-called _order-wire_ system. An order wire, as
ordinarily arranged, is a circuit terminating at one end permanently in
the head receiver of an operator, and terminating at the other end in a
push button which, when depressed, will connect the telephone set of the
operator at that end with the order wire. The operator at the
push-button end of the order wire may, therefore, at will, communicate
with the other operator in spite of anything that the other operator may
do. An order-wire system suitable for transfer switchboards consists in
an order wire leading from each operator's receiver to a push button at
each of the other operator's positions, so that every operator has it
within her power to depress a key or button and establish communication
with a corresponding operator. When, therefore, an operator in a
transfer system answers a call that must be completed through a transfer
circuit, she establishes connection with that transfer circuit and then
informs the operator at the other end of that circuit by order wire of
the number of the trunk and the number of the subscriber with which that
trunk is to be connected. Fig. 334 shows a system of order-wire buttons
by means of which each operator may connect her telephone set with that
of every other operator in the room, the number in this case being
confined to three. Assuming that each pair of wires leading from the
lower portion of this figure terminates respectively in the operator's
talking apparatus of the three respective operators, then it is obvious
that operator No. 1, by depressing button No. 2, will connect her
telephone set with that of operator No. 2; likewise that any operator
may communicate with any other operator by depressing the key bearing
the corresponding number.
=Limitations of Transfer System.= It may be stated that the transfer
system at present has a limited place in the art of telephony. The
multiple switchboard has outstripped it in the race for popular approval
and has demonstrated its superiority in practically all large manual
exchange work. This is not because of lack of effort on the part of
telephone engineers to make the transfer system a success in a broad
way. A great variety of different schemes, all embodying the fundamental
idea of having one operator answer the call and another operator
complete it through a trunk line, have been tried. In San Francisco, the
Sabin-Hampton system was in fairly successful service and served many
thousands of lines for a number of years. It was, however, afterwards
replaced by modern multiple switchboards.
_Examples of Obsolete Systems._ The Sabin-Hampton system was unique in
many respects and involved three operators in each connection. It was
one of the very first systems which employed automatic signaling
throughout and did away with the subscribers' generators. It did not,
however, dispense with the subscribers' local batteries.
Another large transfer system, used for years in an exchange serving at
a time as many as 5,000, was employed at Grand Rapids, Michigan. This
was later replaced by an automatic switchboard.
[Illustration: Fig. 335. Three-Position Transfer Switchboard]
=Field of Usefulness.= The real field of utility for the transfer system
today is to provide for the growth of simple switchboards that have
extended beyond their originally intended limits. By the adding of
additional sections to the simple switchboard and the establishment of a
comparatively cheap transfer system, the simple boards may be made to do
continued service without wasting the investment in them by discarding
them and establishing a completely new system. However, switchboards are
sometimes manufactured in which the transfer system is included as a
part of the original equipment. In Fig. 335 is shown a three-position
transfer switchboard, manufactured by the Monarch Telephone Company. At
first glance the switchboard appears to be exactly like those described
in Chapter XXI, but on close observation, the transfer jacks and signals
may be seen in the first and third positions, just below the line jacks
and signals. There is no transfer equipment in the second position of
this switchboard because the operator at that position is able to reach
the jacks of all the lines and, therefore, is able to complete all calls
originating on her position without the use of any transfer equipment.
Referring to Fig. 301, which illustrates a two-position simple
switchboard, it may readily be seen that if the demands for telephone
service in the locality in which this switchboard is installed should
increase so as to require the addition of more switchboard positions,
this switchboard could readily be converted to a transfer switchboard by
placing the necessary transfer jacks and signals in the vacant space
between the line jacks and clearing-out drops.
[Illustration: CABLE TURNING SECTIONS, BETWEEN A AND B BOARDS Cortlandt
Office, New York Telephone Co.]