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BELL SYSTEM TECHNICAL JOURNAL 



.3 of a milliampere into 600 ohms impedance, or 50 microwatts. It 

 was then given a preliminary stage of ampHfication (ampHfier No. 1, 

 Fig. 4), passed through the line filter No. 3 (Fig. 7) and separated into 

 2125 cycles and 760 cycles modulated at 17.7 cycles. The 2125- 

 cycle component was then amplified by two stages of amplification 

 (amplifier No. 2) ending in push-pull 50-watt tubes and applied to 



—I— — ^ 760~ 



^^ 



Fig. 7- — Line filters for synchronizing frequencies 



the high frequency motor. These amplifiers being of the standard 

 type are not described. The terminal voltage on the output coil of 

 the amplifier was made greater than that of the high frequency motor 

 so that the power flow was normally from the amplifier to the motor. 



®0 O"''®^ A+I20V 

 I 3 



Fig. 8 — Synchronizing demodulator 



The anti-hunting condenser was retained between the amplifier and 

 the motor. 



In the case of the low frequency circuit the output from line filter 

 No. 3 was received in the form of 760 cycles modulated at 17.7 cycles. 

 This was passed through the demodulator (Fig. 8) which operated a 

 polarized relay whose armature opened and closed its contacts at 

 17.7 cycles per second. The contacts of the relay provided square- 

 wave low frequency current by interrupting power from a local battery 



