Recently this company has perfected a system wherein no ground is required at the subscriber's station and no ground return path is used for any purpose between the subscriber and the central office. This later system is known as the "two-wire" system, and in contra-distinction to it, the earlier and most used system has been referred to as the "three-wire." It is not meant by this that the line circuits actually have three wires but that each line employs three conductors, the two wires of the line and the earth. The three-wire system will be referred to and described in detail, and from it the principles of the two-wire system will be readily understood.

Fig. 382. Automatic Wall Set
[View full size illustration.]

Fig. 383. Automatic Desk Stand
[View full size illustration.]

Subscriber's Station Apparatus. The detailed operation of the three-wire system may be best understood by considering the subscriber's station apparatus first. The general appearance of the wall set is shown in Fig. 382, and of the desk set in Fig. 383. These instruments embody the usual talking and call-receiving apparatus of a common-battery telephone and in addition to this, the signal transmitter, which is the thing especially to be considered now. The diagrammatic illustration of the signal transmitter and of the relation that its parts bear to the other elements of the telephone set is shown in Fig. 384. It has already been stated that the subscriber manipulates the signal transmitter by rotating the dial on the face of the instrument. A clearer idea of this dial and of the finger stop for it may be obtained from Figs. 382 and 383.

Fig. 384. Circuits of Telephone Set
[View full size illustration.]

Operation. To make a call for a given number the subscriber removes his receiver from its hook, then places his forefinger in the hole opposite the number corresponding to the first digit of the desired number. By means of the grip thus secured, he rotates the dial until its movement is stopped by the impact of the finger against the stop. The dial is then released and in its return movement it sends the number of impulses corresponding to the first digit in the called number. A similar movement is made for each digit.

In Fig. 384 is given a phantom view of the dial, in order to show more clearly the relation of the mechanical parts and contacts controlled by it. For a correct idea of its mechanical action it must be understood that the shaft 1, the lever 2, and the interrupter segment 3 are all rigidly fastened to the dial and move with it. A coiled spring always tends to move the dial and these associated parts back to their normal positions when released by the subscriber, and a centrifugal governor, not shown, limits the speed of the return movement.