194 



BELL SYSTEM TECHNICAL JOURNAL 



the same positions with respect to the stators it is evident that the 

 e.m.f.'s produced in them by transformer action will be identical as 

 to voltage and phase. Consequently there will be no flow of current 

 over the rotor leads and hence no torque developed. If, however, 

 unit A is turned through a small angle then the phase of the e.m.f.'s 

 produced in the rotor circuits will differ from that in B and a current 

 will flow in the rotor circuits producing a torque which will tend to 

 make unit B assume the same position as A. If ^ is rotated con- 

 tinuously B will follow it up to synchronous speed of the stator field 

 at which point the torque will drop to zero since no e.m.f. is induced 

 in the rotor of either machine. 



3 PHASE 



60 CYCLE _ 



Fig. 10 — Electrical driving gear. 



Complete Recording Circuit 



The portion of the circuit shown in Fig. 10, is merely the equivalent 

 of a mechanical gear, neither unit tending to rotate as a motor by 

 itself. In order to produce such rotation, therefore, a distributor 

 set is added as shown in Fig. 11, the distributor acting, to use a 

 mechanical analogy, as the driving gear of the system and each of the 

 individual units of the system as driven gears. The distributor is 

 itself driven by a D.C. motor provided with the speed control circuit 

 previously described and shown in Fig. 9. The speed of the system 

 is thus solely dependent on the D.C. driving motor and independent 

 of the 60-cycle excitation frequency. 



In practice the system is controlled by an operator at the distributor 

 set and by means of switches any desired number of cameras, recording 

 machines, or projectors may be employed. The projecting machines 

 are used in case it is desired to make up a sound record to accompany 

 an ordinary motion picture film which has previously been recorded 

 without sound accompaniment. It has been found that the system 

 operates very satisfactorily and requires very little maintenance. 



