POWER PLANTS FOR TELEPHONE OFFICES 



717 



and a low tone of 160 pulsations per second which are used for various 

 signaling purposes. 



Through a 120/1 worm gear reduction an auxiliary shaft is run at 

 10 r.p.m. Attached to one end of this shaft is a "low-speed inter- 



Fig. 11 — Master control board for all charging sets and main storage batteries. 

 Charging motor-generator in foreground, emergency engine-alternator set in back- 

 ground. Emergency lighting cabinet to left of control board. 



rupter" which provides flashing or tone signals for "busy" and other 

 uses and may provide half a dozen or more different signals. To the 

 other end of this shaft is usually attached a ringing interrupter which 

 divides the constant ringing current from the generator into machine 

 ringing intervals such as 2 seconds ring, 4 seconds silent, or 1 second 

 ring, 1 second silent, 1 second ring, and 3 seconds silent. This inter- 

 rupter also controls battery current for tripping during the silent 

 interval, and a "pickup" circuit the purpose of which is to prevent 

 ringing the wrong party on party lines. 



The generator and all the interrupters are regularly driven by an 

 alternating-current line motor operating upon the outside power 



