The Automatic Telephone The duty of the automatic telephone is twofold, first to talk the same as any telephone, second to operate the switches. The talking part, fig. 460, consists of a transmitter in series with the primary of an induction coil and connected across the line to receive current from the common battery at the central office. Its circuit is controlled by the hook switch as usual. Attached to the secondary of the induction coil is the receiver. The bell and condenser are wired through the hook as usual. The work of controlling the switches is divided into three parts, selecting, ringing, and release. To operate a 3-wire switch requires grounding of each wire a certain number of times. To send the digit "4" we must ground the vertical line wire, V.L., four times and then ground the rotary line, R.L., once. This must be repeated for each digit of the called number, sending over the vertical line as many impulses as there are units in the digit, and following by one impulse over the rotary line. This is done by the impulse springs, actuated by the dial and associated mechanism. During the sending of the impulses it is necessary that each line shall be insulated from each other. Accordingly a finger, D.F., is arranged to cause the springs, 3 and 4, to separate when the dial is turned in calling. When the receiver is on the hook, the ground connection should be cut off from the telephone wiring. This is to enable line and instrument troubles to be more readily separated from each other. This is done by the ground spring, G.S., which is let down on its contact at the first pull of the dial and lifted off when the receiver is placed on the hook. Ringing is accomplished by grounding the vertical line, V.L. The ringing key is wired in series with the talking set and arranged to open the circuit and ground the vertical line when depressed. Release is made by simultaneously connecting both line wires to earth and then clearing them. When the hook lever descends, a lug presses springs 5 and six together and then into contact with spring 7. When the lever is completely down, the lug slips off spring 5, allowing all three springs to clear each other. When the hook lever rises, as in initiating a call, the release springs are not operated, because the lug on the lever passes to the right of spring 5, thereby pressing the latter away from the other springs, instead of against them. Modern American Telephony cc 1911