324 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1954 



USE OF IDLE INDICATING LAMPS AT SWITCHBOARDS 



A system of indicating an idle circuit in a trunk group is used at toll 

 switchboards to relieve the opei-ator of the necessity of making busy 

 tests on the sleeve of the ti'unk. Associated with each trunk jack in the 

 face of the switchboard is a lamp. The lamp associated with the first 

 idle trunk in the group is normally lighted, while those lamps associated 

 with the other trunks in the group remain dark even though those trunks 

 are idle. When the trunk whose idle indicating lamp is lighted is seized, 

 that idle indicating lamp is extinguished and the lamp associated with 



AW 



^ vw 



) AW 



^ Wv- 





(a) INCOMING DIRECTION CONTROL 



(b) OUTGOING DIRECTION CONTROL 



Fig. 12 — Direction of selection control. 



