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MANAGERS AND PROJECTIONISTS 1107
band and, since the film is running steadily at the rate of ninety feet per minute, and since the photographic density of the sound band varies continually in proportion to the sound which caused its impression upon the band, it follows that on the side of the film we now are examining we shall have a beam of light which will vary constantly in brilliance, and will be an exact reproduction of the light beam from the camera slit which made the photographic impression through which it has passed.
The problem now is to transform these light variations back into electric variations the same as those which came from the receiving microphone and operated the Aeo lamp at the camera.
Examining Fig. 385 we find, on the extreme right, "P.-E. Cell,'' which stands for photo-electric cell, which is the device by means of which the light beam variations are transformed into electric vibrations of exactly equal values.
You will find the action of this cell set forth under "The Photo-Electric Cell" (see page 1013), which description coupled with what has been said completes the description of the Western Electric Pick-Up System.
What such systems do in recording and picking up is to receive sound vibrations traveling through the air, change them into electrical vibrations, again change them into light vibrations and photographically impress those vibrations upon the sound track of a film. It then again picks them up in the form of light vibrations, transforms them into electrical vibrations, amplifies them approximately 100,000,000 times and changes them back into an exact duplication of the original air vibrations which set the receiving or recording system into motion.