SYNCHRONIZED SOUND PICTURES 193 



This causes a current to flow through the coupling resistances i?i and 

 Ri which drives the grids of tubes Vi and Fo negative, thereby keeping 

 down the current through these tubes and hence maintaining a weak 

 motor field. The motor, therefore, continues to accelerate until a 

 speed of 1,200 R.P.M. is reached. At this point as previously explained 

 under the description of the bridge circuit, the phase of the output 

 suddenly reverses whereupon the grid of the detector tube goes 

 negative at the same time that the plate goes positive, thereby cutting 

 off the current through the detector tube Vz and reducing the negative 

 C voltage on the grids of tubes Vi and V^. This increases the plate 

 current through the regulating field thereby stiffening the field of 

 the motor and checking its rise in speed. In practice the current 

 through the detector tube is neither at one extreme nor the other but 

 reaches an equilibrium at the speed of 1,200 R.P.M. A feedback 

 network having the delay feature for prevention of hunting is included 

 in the same manner as previously described for the A.C. circuit. The 

 characteristic curves for the D.C. motor are similar to those shown in 

 Fig. 7 for the A.C. motor. 



For the operation of ordinary motion pictures the motor is changed 

 to a simple shunt D.C. motor by the switch Si and the speed varied 

 by means of the field rheostat. 



Motor Drive of Recording System 



It might appear that the simplest method of securing synchronization 

 in recording work would also be mechanical connection between the 

 recording machine and the camera. It has been found desirable, 

 however, from a practical standpoint to have the camera movable 

 with respect to the recording machine as the recorder has to be 

 accurately lined up and adjusted and is not essentially a portable 

 machine whereas the camera in ordinary motion picture work must be 

 a portable piece of equipment. It has been necessary, therefore, to 

 develop a motor drive equipment which will satisfactorily interlock 

 the camera and the recording machine but leave the camera unit 

 portable. It is essential that the interlock should hold not only 

 during normal conditions but during acceleration and deceleration. 

 In other words, the system must be the full equivalent of a mechan- 

 ically geared system. The principle employed is old being disclosed 

 in a patent issued to Michalke in 1901. In Fig. 10, A and B are two 

 units which it is desired to interlock. Each unit has a three phase 

 stator and a three phase rotor, the latter provided with slip rings. 

 Magnetizing current for the system is supplied from an independent 

 three phase, 60-cycle source. If the rotors of A and B are in exactly 



