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BELL SYSTEM TECHNICAL JOURNAL 



supply. In addition to this, however, the set includes a direct- 

 current motor designed to operate on current from the central office 

 battery but normally not connected to the battery. Automatic relays 

 and magnetic switches close the connection to the battery when the 

 regular power fails so the set continues to operate without interruption 



Fig. 12 — Rear of control panel for charging sets and batteries. 



or change in output. This feature avoids the delay of a few minutes 

 which would otherwise result after a power failure, while the attendant 

 started up a reserve motor-generator and transferred the load circuits, 

 which may run from 17 circuits to twice that number for ringing, coin 

 control, tones and signals. It also avoids accumulation of machine 



