Battery Supply to Connected Subscriber. Throughout the whole process of building up a connection, it will be remembered that both sides of the calling line are connected through the respective vertical and rotary relays involved in building up the connection with the live side of the battery. At the time when the connection is finally established and the called subscriber rung, both sides of the calling line are connected through various relay windings to the live side of the battery. Such a condition leaves both sides of the line at the same potential and, therefore, there is no tendency for current to flow through the calling subscriber's talking apparatus, even though it is connected across the circuit of the line. It remains, therefore, to be seen how these conditions are so changed after the building up of a connection as to supply the calling subscriber with talking current.

The calling subscriber can get no current until the called subscriber responds. When the connection is first made with the called subscriber's line, battery connection to his line is made from the live side of battery through the normally closed contacts of the calling battery supply relay, thence through the winding 25 of the called battery supply relay to the vertical side of the called line. The grounded side of the battery is connected to the rotary side of his line through the third wiper of the connector and the coil 26 of the called battery supply relay. As a result, this subscriber receives proper talking current through the coils 25 and 26, and this relay is operated by the flow of this current. The operation of this called battery supply relay merely shifts the connection of the rotary side of the calling subscriber's line from its normal battery connection, to ground, and thus the battery is placed straight across the calling subscriber's line so as to supply talking current. This supply circuit to the calling subscriber may be traced from the live side of the battery through the winding 13 of the calling battery supply relay and the winding of the vertical relay to the vertical side of the line, and from the grounded side of battery through the third side switch wiper in its third position to the now closed pair of contacts in the called battery supply relay through the coil 18 of the calling battery supply relay and the coil of the rotary relay to the rotary side of the line.

It will be noted that the system of battery supply is that of the standard condenser and retardation coil scheme largely employed in manual practice; and that aside from the coils through which the battery current is supplied to the connected subscribers, there are no taps from, or bridges across, the two sides of the talking circuit.

Release after Conversation. It remains now only to secure the disconnection of the subscribers after they are through talking. When the calling subscriber hangs up, the whole disconnection is brought about, all of the apparatus, including connector, selectors, and line switch, returning to normal. This is done by the back release system and is accomplished in almost the same way as has already been described in connection with the disconnect after an unsuccessful call. There is this difference, however: after an unsuccessful call when the line called for was found busy, the release was made while the connector side switch was in its normal position. In the present case, the release must be made with the connector side switch in its third position and with the talking battery bridged across the metallic circuit rather than connected between each limb of the line and ground. It must be remembered that the calling battery supply relay, while traversed by current during the conversation, is not magnetically energized because, with the current flowing through the metallic circuit of the line, the two windings exert a differential effect. As soon, however, as the calling subscriber hangs up his receiver, this differential action ceases, due to the grounding of both sides of the line at the subscriber's station. This relay, therefore, operates and cuts off battery from the called battery supply relay and this, in turn, releases its armature and thus changes the connection of the rotary side of the calling line from ground to live side of the battery. The normal condition of the battery connection now being restored, both the vertical and the rotary relays at the connector become operated, due to the ground on both sides of the line at the subscriber's station, and this, as we have seen, is the condition which brings about the operation of the connector release magnet, and the relaying back of the disconnect impulse successively through the selectors to the line switch.

Multi-Office System. In exchanges involving more than one office, the same general principles and mode of operation already outlined apply. If the total number of subscribers in the multi-office exchange is to be less than ten thousand, then four digit numbers suffice, and the first movement of the dial may be made to select the office into which the connection is to go, the subscribers' lines being so numbered with respect to the offices that each office will contain only certain thousands. The choosing of the thousand by the calling subscriber, therefore, takes care in itself of the choice of offices. Where, however, a multi-office exchange is to provide for connections among a greater number of lines than ten thousand and less than one hundred thousand, then it will take five movements of the dial to make the selection—the five movements corresponding either to the five digits in a number or to the name of an office, as indicated on the dial, and the four digits of a smaller number. The lines may all carry five digit numbers or, what is considered better practice, may be designated by an office name followed by a four digit number. In this latter case the numbers of the subscribers' lines will in each case be contained in one or more of the tens of thousands groups, no number having more than four digits. And the first movement of the dial, whether the name or number plan be adopted, will select an office; or, looking at it another way, will select a group of ten thousand and this being done, the next four successive movements of the dial will select the numbers in that ten thousand in just the some way that has been already described.

Certain difficulties arise, however, in multi-office working due to the fact that the three-wire trunks between offices would in most cases be objectionable. As long as the trunks extend between the various groups of apparatus in the same office, it is cheaper to provide three wires for each of them than it is to make any additional complication in the apparatus. Where the trunking is done between offices, however, the system may be so modified as to work over two wire inter-office trunks.

The Trunk Repeater. The purpose of the trunk repeater is to enable the inter-office trunking to be done over two wires. It may be said that the trunk repeater is a device placed in the outgoing trunk circuit at the office in which a call originates, which will do over the two wires of the trunk leading from it to the distant office just the same thing that the subscriber's signal transmitter does over the two wires of the subscriber's lines. It has certain other functions in regard to feeding the battery for talking purposes back to the calling subscriber's line, taking the place in this respect of the calling battery feed relay in the connector in a single office exchange.

Fig. 397. Circuits of Trunk Repeater
[View full size illustration.]

The circuits of a trunk repeater are shown in Fig. 397. In considering it, it must be understood that the three wires entering the figure at the left are the vertical, rotary, and release wires of a second selector trunk leading from the first selector banks in the same office. The two wires leading from the right of the figure are those extending to the distant office, and terminate there in second selectors. The vertical and the rotary sides of this trunk as shown at the left will receive the impulses from the subscriber's station coming through the line switch and the first selector, as usual. The vertical impulses will pass through the winding of the vertical relay and through the winding 1 of the calling battery supply relay and thence to battery, the same as on a connector. These impulses will work the armatures of both of these relays in unison. The movements of the vertical relay armature in response to these impulses will cause corresponding impulses to flow over a circuit which may be traced from ground, through the springs 3 and 2 of the vertical relay, the springs 4 and 5 of the bridged relay 6 and thence to the vertical side of the trunk and to the distant office, where it passes into a second selector and through its vertical relay to battery. Thus the vertical impulses are passed on over the two-wire trunk to the second selector at the distant office. It becomes necessary, however, to prevent these impulses from passing back through the winding of the bridge relay 6 and this is done by means of the sluggish relay 7. This relay receives local battery impulses in unison with those sent over the trunk by the vertical relay, these being supplied from the battery at the local office through the contacts 8 and 9 of the calling battery supply relay, which works in unison with the vertical relay. These rapidly recurring impulses are too fast for the sluggish relay 7 to follow. And this relay merely pulls up its armature and cuts off both sides of the trunk leading back to the first selector. The rotary impulses are repeated to the rotary side of the two-wire trunk in a similar way.