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1146 HANDBOOK OF PROJECTION FOR
cell employed by RCA is a small amount of inert gas, as in the other. The cell is energized by a 200-volt D. C. battery, or its equivalent. This battery, or its equivalent, which in certain RCA Photophone installations is a motor generator set, see Fig. 418, is connected to the cell in series with the primary winding of a 12-to-l stepdown transformer.
The light fluctuations entering the photo-electric cell cause it to create a pulsating current (see pages 1013 to 1017) in the circuit, which same includes the 200-volt supply current and the primary of the transformer. This current energizes the transformer, causing it to step down, or reduce the voltage, bringing it down to onetwelfth (1/12) of its former value.
This step-down obviates the necessity for mounting a vacuum tube amplifier in the photo-electric cell compartment, or adjacent thereto, in order to eliminate the chance of microphonic noises affecting the extremely weak electrical impulses emanating from the photoelectric cell, which same might be set up by vibration of the projector mechanism.
This stepped-down current then is passed to the fader (which must not, in RCA apparatus, be confused with the volume control) and thence to the primary of another transformer — a 1 to 12 this time — mounted upon the main amplifier panel, which steps the voltage up again to its original value as it came from the photoelectric cell.
In brief, then, the entire sound reproduction system consists, in so far as has to do with its main parts, in (a) an exciting lamp; (b) a slit; (c) a slit optical system; (d) a film sound track; (e) a photo-electric cell; (f) a 200-volt storage battery or its equivalent;