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MANAGERS AND PROJECTIONISTS 1013
Sound Reproduction Apparatus
THE PHOTO-ELECTRIC CELL.— In all systems of sound recording and reproduction in which the sound is photographically impressed upon the sound track of a film, the photo-electric cell plays a most important part. It is this marvelous instrument, the product of man's ingenuity, which receives the light beam emanating from the exciting lamp after it has passed through the film sound track, and transforms the variations in its brilliancy set up by the said sound track back into a duplication of the electrical impulses which controlled the variations of light brilliancy by which the film sound track was originally made.
In Fig. 387 we have a view of this remarkable device, the location of which in the pick-up system of the motion picture projector is indicated by "P. E. cell" in Fig. 405.
The "cell" consists of a glass bulb or "tube" of the form shown, the interior wall of which, all but a circular opening on one side, is coated with a thin deposit of silver electrically connected wTith a wire which enters the bulb through an air-tight seal. This is the negative wire. In the center of the bulb we see a ring-shaped wire connecting with the positive wire, which also enters the bulb through an air-tight seal.
On the surface of the silver deposit described, is another deposit, the silver really merely serving to form a connection between it and the negative wire. It is composed of a special form of metallic potassium, usually