SYNCHRONIZED SOUND PICTURES 



187 



the outer legs thereby reducing their impedance to A.C. This type of 

 reactance is old and was employed by Alexanderson as a magnetic 

 modulator in his early radio work. 



AUXILIARY 

 ALTERNATOR 



110 VOLT 



60 CYCLE 



LINE 



Fig. 2 — A. C. Motor circuit diagram. 



Fig. 3 shows one element of the speed control circuit. This consists 

 of a bridge circuit having one variable arm and three fixed arms. 

 The variable arm comprises a tuned circuit consisting of the in- 



Eo 



Eo 



Fig. 3 — Bridge circuit in speed control system. 



ductance L and the capacity C which are designed to tune at exactly 

 the frequency corresponding to the desired motor speed. When this 

 circuit is in tune it has a resistive impedance which is balanced by the 

 fixed arm R of the bridge. The other two fixed arms on the left 



