THE TELEPHONE AND TELEGRAPH 217 



case, the trunk line operator would call this apportioning 

 operator instead of calling operator B directly. 



263. The Supervising Lamps. There is, on the operator's 

 switchboard, a supervising lamp associated with the calling 

 plug as well as with the receiving plug. When a subscriber 

 calls the operator, the supervising lamp under the receiving 

 plug lights, as before noted. When the operator " plugs in" 

 and connects herself with the calling subscriber, that lamp 

 is extinguished. When she " plugs in" at the calling hole, 

 the second supervising lamp lights, and remains lighted until 

 the party called answers. So long as these two lights are 

 extinguished, the operator knows that the subscribers are 

 using the line. When the subscribers replace their receivers 

 on the hooks, the lamps relight. 



264. The Listening Cam. The listening cam is a small 

 key on the switchboard by means of which the operator puts 

 herself in connection with a subscriber after having ''plugged 

 in" at his answering jack. After connecting two subscribers 

 the operator closes her listening key and thus shuts herself 

 off from their conversation. Were it not for this device, 

 every conversation would, perforce, pass through the opera- 

 tor's ears. 



265. Cables and Distributing Frames. Wires enter and 

 leave the telephone exchange building in the form of cables 

 (Fig. 95). A cable is composed of pairs of twisted copper 

 wires, insulated with spiral wrapping and enclosed in a lead 

 casing. 



Within the exchange these cables are supported by two 

 frames; the main distributing frame and the intermediate 



