. American telephone practice . FIG. 172.—CIRCUIT OF COMBINED RINGING AND LISTENING KEY. springs 1 and 2, and also to the springs 3 and 4. Normally restingagainst the springs 3 and 4 are the springs 5 and 6, which are per-manently connected with the tip and sleeve strands, respectively, ofthe calling cord. The springs 7 and 8 form the terminals of theoperators telephone set, while the springs 9 and 10 form the termi- MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 191 nals of the calling generator. It will be seen that the tip strand isnormally continuous from the tip of the plug, P, to the tip of t
. American telephone practice . FIG. 172.—CIRCUIT OF COMBINED RINGING AND LISTENING KEY. springs 1 and 2, and also to the springs 3 and 4. Normally restingagainst the springs 3 and 4 are the springs 5 and 6, which are per-manently connected with the tip and sleeve strands, respectively, ofthe calling cord. The springs 7 and 8 form the terminals of theoperators telephone set, while the springs 9 and 10 form the termi- MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 191 nals of the calling generator. It will be seen that the tip strand isnormally continuous from the tip of the plug, P, to the tip of theplug, P, through the springs 3 and 5 of the ringing key; likewisethe sleeve strand is normally continuous from the springs 4 and 6of this key. If the ball cam, A, is pressed to the left by means ofthe cam handle the operators telephone is bridged across the cir-cuit by causing the springs I and 2 to make contact with the springs7 and 8, respectively. If the ball cam is pressed to the right the. FIG. OF LISTENING KEY. connection between the plug P and P is entirely severed by thebreaking of the contact between the springs 3 and 5 and 4 and 6respectively. A little later in the movement of the cam the springs5 and 6 make contact with the springs 9 and 10, thus connecting thegenerator terminal with the tip and sleeve of the calling plug. Thereason for breaking the connection between the two plugs in theringing operation is to prevent the current from the calling gener-ator also being sent out on the line of the calling subscriber, withwhich the plug, P, is connected. If this happened the current wouldbe likely to traverse the receiver coil held to the ear of the waitingsubscriber, giving him what is commonly known as a ring in theear, a decidedly unpleasant and sometimes dangerous diagrammatic illustrations of cord circuits it is usually more con-
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