. American telephone practice . FIG. 173.-DIAGRAM 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 th

. American telephone practice . FIG. 173.-DIAGRAM 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 th Stock Photo
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Reading Room 2020 / Alamy Stock Photo

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2CRX7CA

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2647 x 944 px | 22.4 x 8 cm | 8.8 x 3.1 inches | 300dpi

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. American telephone practice . FIG. 173.-DIAGRAM 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 experience.In diagrammatic illustrations of cord circuits it is usually more con-. FIG. 174.—DIAGRAM OF RINGING KEY. venient to show the ringing and listening keys separately, althoughthey may, in fact, form virtually one piece of apparatus. Thus Fig.173 would represent the listening key, and Fig. 174 the ringing key.It is also very common in circuit diagrams to omit entirely the ballcam, and also to omit the supporting blocks upon which the springsare mounted. It may be said in general that in diagrammatic illustration of tele-phone circuits the details of mechanical construction must often be 192 AMERICAN TELEPHONE PRACTICE. sacrificed entirely to clearness, in order to represent the circuit in anintelligible manner. The telephone circuit is often such a compli-cated thing that it should not be required to carry with it anydegree of accuracy as to mechanical construction or arrangement. It is frequently desirable to provide in connection with the cordcircuit what is commonly called a ring-back key, by means of whichthe operator may send a calling current out on

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