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4& SOUND MOTION PICTURES
width of the transparent part of the marked track. In the same manner is regulated the light which passes through the photograph film.
The R. C. A.-Photophone does not use a diaphragm. The sound comes through paper cones which act in a measure in place of a diaphragm. These are made in two sizes, twelve or sixteen inches in diameter and about six inches deep. They taper to an opening about the size of a silver dollar. At the outer circumference the horn is mounted on soft flexible kid leather, and on the inside on three fine silk threads. The horns are placed around the screen and masked from view. The number employed is determined by the size of the auditorium.
The Photophone method is to a great extent similar to that of Movietone. It has added to the projector a "sound head," which enables a narrow ray of light to be thrown on the sound track on the film. The light, after penetrating the sound track on the film, then falls on the sensitive photo-electric cell, which produces electrical currents corresponding to those of the original sound. An amplifier in the projection booth increases the power of the current thus formed, which is then transmitted by wire to the loudspeakers around the screen.
The component elements of a Photophone film are shown in Plate I. On the lower edge of this illustration is shown the type of sound record used by R. C. A.-Photophone. It will be seen that this is a narrow black band, the inner edge of which (that nearest to the picture) has a wavy outline. This last constitutes a faithful record of the air vibrations in the original music or speech. Its minute and complex contours are literally the permanent and accurate signature of the original sound waves. The combined picture and sound record shown demonstrates how picture and sound are bound together on every R. C. A.-Photophone film.