556 BELL SYSTEM TECHNICAL JOURNAL 



These tubes act as a phase detector and vary the input voltage across 

 an ampHfier which supplies the total armature current of the motor. 

 Armature rather than field control is employed to obtain faster and 

 more complete regulation. The capacitor across the terminals of the 

 generator armature, tuning the circuit to a frequency slightly in 

 excess of 300 cycles per second, and the coupling impedance between 

 phase detector and amplifier acting as a low-pass filter, assist in 

 preventing hunting of the motor. 



Clutch 



Connection between driving motor and picture machine is made 

 through a positive action clutch electrically operated. This clutch 

 gradually applies the driving torque to the picture machine during 

 the starting interval. It operates on the principle of storing energy 

 in a coiled spring during the first part of the starting interval while 

 the velocity of the machine is increasing and then allowing this energy 

 to be released gradually by an escapement mechanism while the parts 

 of the clutch are assuming their normal operating position. The 

 time interval required for complete operation of the clutch corresponds 

 to three or four revolutions of the picture cylinder but variations in the 

 length of this interval do not affect the accuracy of phasing, inasmuch 

 as the latter is determined by the time of operation of the clutch trip 

 magnet and each receiving machine may be readily adjusted to 

 compensate for this variation. 



Circuit arrangements associated with the clutch of the receiving 

 machine permit its operation from a starting signal received over the 

 line from the transmitting machine. 



Sending Optical System 

 The optical system of the sending machine is arranged to direct a 

 scanning light beam upon the surface of the picture which is mounted 

 on a cylinder. This scanning beam, attenuated by reflection from the 

 various shades of the picture, is directed to a photoelectric cell. The 

 illumination is obtained from a small incandescent lamp and is inter- 

 rupted in passing through the aperture of a double ribbon light valve. 

 This double ribbon light valve, which is a modification of a type 

 previously described,* is actuated by the picture carrier frequency, 

 2,400 cycles per second, and its interruption of the scanning light beam 

 permits the use of a-c. methods of amplification of the photoelectric 

 currents. Aside from its general simplicity and freedom from the 

 usual difficulties experienced with rotating light choppers, this type of 

 interrupter readily effects a sinusoidal variation in illumination. 



