608 BELL SYSTEM TECHNICAL JOURNAL 



the variations from one d-c. machine to the other to hold synchronism 

 in a completely two-way system. As was to be expected, it was 

 found that the motors hunted badly at a frequency of about four 

 cycles per second. In other words, instead of holding within a fixed 

 electrical phase angle of 20 degrees the receiving motor oscillated 

 throughout a phase angle of about ± 20 electrical degrees. This, 

 of course, made the picture wabble back and forth across the aperture 

 and was therefore unsatisfactory. 



The ordinary method of preventing hunting by means of copper 

 bars embedded in the pole faces was not practical on account of the 

 large number of poles and limited space. The hunting trouble was 

 cured by employing a series condenser between the motors using a 

 value of capacity somewhat less than that required to tune the circuit. 

 A rigid analytical treatment of this anti-hunting circuit is beyond the 

 scope of this paper but its operation depends in general upon the 

 curvature of the tuning curve due to the variation of the inductance 

 of the machine with phase displacement. Since the condenser operates 

 on the total inductance of the circuit, it is desirable to make the 

 natural periods of oscillation of the two motors different. Otherwise 

 a decrease in the inductance of one machine may be accompanied by 

 a simultaneous and equal increase in the inductance of the other, thus 

 leaving the total inductance unchanged and preventing the condenser 

 from functioning. This was done by making one disk substantially 

 heavier than the other. 



The series condenser also neutralizes the greater part of the internal 

 reactance of the motors, thereby increasing the steady state torque. 



Framing of Picture 



There was still one unsatisfactory feature in this system in that the 

 motor at the receiving end could interlock in any one of 120 different 

 angular positions whereas in order to get proper framing of the picture 

 it must be synchronized at a particular angular position. For example, 

 if the disk at the receiving end is exactly 180 degrees out with respect 

 to the disk at the transmitting end, the observer will see the lower half 

 of the picture on top; a dark space representing the dividing line 

 between pictures and the upper half of the picture at the bottom. 

 Similarly, if the disk is 90 degrees out at the receiving end, the lower 

 quarter of the picture will appear on the top and the upper three 

 quarters of the picture on the bottom. The disk at the receiving end 

 may be brought into correct angular position by providing means for 

 turning the entire motor through the necessary angle. It was found, 

 however, that the rate at which the motor can be turned was limited 



