The art of sound pictures (1930)

232 THE ART OF SOUND PICTURES into an electric current. Our light waves, which originated in the photographic sound record, must then be transformed, first into electric currents, and these electric currents, in turn, may be transformed into air waves of sound. A wholly different type of apparatus from that of recording is required in solving the problem with which we are now dealing. A small photo-electric cell is the device used to perform the first of these steps. A photo-electric cell is a piece of metal fastened inside of a vacuum tube. Certain metals are selected for this purpose because of their peculiar ability to give off electric currents when light of a certain sort is focused upon them. The metals used are selenium, sodium, and potassium. The reason sodium and potassium are enclosed in vacuum tubes is that these metals become chemically active when permitted contact with the oxygen in ordinary air. Thus, we see that the photo-electric cell is essentially a simple device which utilizes the natural properties of certain metals to translate sound waves of air into electron waves of the electric current. When a beam of light falls upon the photo-electric cell, the cell immediately sets up a minute electric current. These photo-electric currents are so tiny that they cannot be used in any practical way without amplification. Therefore, audion tubes similar to those in our ordinary radio sets are put into the circuit. The audion tube simply catches the photo-electric currents and amplifies or increases them to a strength necessary to operate sound-making devices in the loud speaker. As many audion tubes may be put into the circuit as seems desirable. The more the photo-electric current is amplified, of course, the more strenuously the electric sound-making