SYNCHRONIZATION OF TELEVISION 



613 



source. On account of the limited power output which the vibrating 

 contacts could safely handle without sparking, it became necessary 

 to amplify this low frequency output. While this would have been 

 possible by the use of ordinary amplifier circuits, it was found pre- 

 ferable from the standpoint of economy of apparatus to apply the 

 low frequency regulation through a field circuit of the receiving motor. 

 Referring to Fig. 9 it will be noted that the plate circuit of the reg- 



REGULATING FIELD 

 OF MOTOR 



Fig. 9 — Low frequency regulator 



ulating tubes is supplied from the secondary of the transformer which 

 is connected to the slip rings of the motor, while the grid circuit of 

 these tubes is supplied with low frequency, low power 17.7 cycles from 

 the contacts of the relay. As the motor is started up from rest the 

 shunt field is weakened until the motor falls in step. At this 

 point the frequency of the plate supply to the regulator tubes 

 is identical with that supplied to the grids. If the phase relationship 

 is such that the plates go positive at the same time that the grids are 

 positive, then the space current of the tubes is increased and the 

 regulating field (which is an aiding auxiliary field) is strengthened, 

 thereby preventing a further rise in the speed of the motor. In other 

 words, for each combination of load and line voltage there is an 

 equilibrium phase position between the plate and grid voltages at 

 which the corresponding regulating field current maintains the speed 

 at the desired value. 



Motor Operation 



In actual operation the procedure was to first synchronize on the 

 low frequency, and then on the high frequency circuit. The precise 

 framing of the picture was then adjusted by rotating the motor by 

 means of worm gearing through the necessary angle to center the 

 image properly in the aperture. The high frequency current was of 

 the order of 1.5 amperes at 2125 cycles with a terminal voltage of 



