A NEW TELEPHOTOGRAPH SYSTEM 559 



currents. A uniformly illuminated field is obtained at the light valve 

 aperture with a minimum loss of light by using a sphero-cylindrical 

 condenser lens which focuses the diameter of the helical filament of the 

 lamp without imaging individual turns of the helix at the plane of the 

 aperture. Imaging of the lamp filament with the usual type of 

 spherical condenser lens would result in non-uniformity of illumination 

 not only because interstices between individual turns of the helix 

 have an intrinsic brilliancy much greater than the outer surface of the 

 filament but also because of the angular variation of the masking 

 effect of the turns of the helix upon the illumination emerging from 

 the interstices. The ribbon of the light valve is tuned mechanically 

 to resonance at 1,200 cycles per second, and is shunted by an equalizer ^ 

 consisting of inductance, capacitance, and resistance in series which is 

 tuned to the resonant frequency of the ribbon, thereby producing a 

 flat response-frequency characteristic over the useful range of signal 

 frequencies. 



Carrier and Motor Control Oscillator 

 This portion of the equipment furnishes the carrier frequency of 

 2,400 cycles per second and the motor control frequency of 300 cycles 

 per second accurate to within a few parts in a million. The arrange- 

 ments used consist of a 300-cycle tuning fork within a temperature 

 regulated container, a vacuum tube amplifier circuit designed to 

 provide controlled regenerative operation of the fork, and a vacuum 

 tube harmonic generator for supplying the carrier frequency. 



Although this general method for obtaining a constant frequency 

 is old and has been described previously,^- ^- ^^ in view of its importance 

 in the operation of the present telephotograph equipment it may be 

 of interest to indicate briefly the specific arrangements employed. 



The tuning fork is made of a heat treated nickel chromium steel 

 alloy to obtain a small frequency-temperature coefficient and is 

 mounted in a thermostatically controlled metal cylinder wound with 

 a heating coil over which are wrapped alternate layers of copper and 

 felt to provide attenuation of heat transfer. ^^ The pick-up and drive 

 coils associated with the fork are connected to the vacuum tube 

 amplifier circuit as shown in Fig. 7. The frequency of a fork is 

 affected by a number of factors including temperature, amplitude of 

 vibration, and aging of the material. Since it is impracticable to 

 maintain constant all of the factors involved, it is necessary to provide 

 means for occasional adjustment to meet the requirements for con- 

 stancy desired in picture transmission. In the present equipment the 

 temperature of the fork is maintained within ±0.1 degree of its 

 nominal value of 50 degrees centigrade; two adjustments are provided 



