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44 SOUND MOTION PICTURES
camera is motor driven and is connected by wire with the telephonic apparatus, there is little difference between the recording of picture and voice by this method and the ordinary picture recording in a studio. This is called the "flashing lamp" method of recording.
The presentation of one of these sound pictures to an audience is in effect a reversal of this process. The standard film, containing both picture and sound in a photographic record, is run through a standard projection machine to which has been attached a sound reproducing unit. This unit includes a light that is reflected onto the sound record of the film. As the sound record on the film passes the light it interrupts the constant light shining through it and sets up light variations corresponding to those photographed. These variations then fall on a photo-electric cell which changes the light variations back to electrical vibrations. The latter variations, being too delicate to be heard directly, are accordingly amplified and carried by wire from the projection booth to the horns placed back of the screen, from which the sound emerges.
A main amplifier raises the energy of the feeble electric currents to a level adequate to supply the loudspeakers with sufficient volume to serve the specific theatre. As a step to follow the amplifier, there is an output control panel which consists of an auto-transformer, with a large number of taps. Each of the sound projectors or loudspeakers is connected to a dial switch, and these dial switches in turn are connected to the taps, so that each sound projector can be fed from any tap desired. Through this panel it is possible to make the impedance of the amplifier output suitable for the desired number of horns, in order to obtain the most efficient use of the power available, and also to adjust the relative volume of the individual horn.
It will therefore be seen that the only difference between