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BELL SYSTEM TECHNICAL JOURNAL 



of two motor elements in one frame, one element being normally 

 driven from the commercial power service and the other being driven 

 by the telephone reserve storage battery to which it is automatically 



Fig. 14 — Group of Typical Selector Frames 



connected by a relay inside the motor when the regular power fails. 

 A power failure, therefore, causes no interruption to the drive. The 

 selectors are arranged so that not more than half in any one group are 



