538 BELL SYSTEM TECHNICAL JOURNAL 



provided with a holding- contact so that after the selector has been 

 restored to normal by the releasing impulse, it will remain operated 

 until the person at the receiving station presses the key which opens 

 the circuit of the holding contact. In the operated position, the 

 relay completes the circuit of the red signal lamp on the panel of the 

 receiver, thus indicating to the operator that he has been called. An 

 audible signal may also be connected in parallel with it, using an 

 additional relay if necessary to handle the heavy current required 

 by a large gong. 



In the exterior views of the receivers at the extreme left of the 

 receiver is shown a jack into w^hich the head telephones may be plugged. 

 Plugging in these telephones does not interfere in any way with the 

 operation of the signal system. Just to the right of the telephone 

 jack is shown the red signal lamp for indicating when the receiver 

 has been called. In the middle of the panel is a key which is used 

 by the operator of the receiver to extinguish the red signal lamp 

 when he takes up his head telephones. To the right of the key is 

 the green signal lamp indicating when the pow'er is on the receiver, 

 and to the extreme right is the switch for turning the power on and off. 



