CHAPTER XXIII

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.

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.

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.

_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.

=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.

=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.