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MANAGERS AND PROJECTIONISTS 997
ation in voltage, frequency or load will affect both exactly alike. And that is how it is done.
Two synchronous motors are employed, one to drive or pull the camera, and one the sound recorder. Each motor is built to operate on the same current frequency, and is geared to move exactly ninety feet of film past the aperture each minute. The plan is simple and highly effective.
THE SOUND RECORDER.— The sound recorder consists of three main parts, namely an oscillograph galvanometer, an optical system and a motor-driven mechanism to move the film past the recording light beam. There are of course the usual light-tight magazines, sprockets, etcetera, plus a rather heavy flywheel to insure perfect constancy of movement.
The recording of sound is accomplished substantially as follows : The sound waves strike the microphone diaphram, and in so doing effect a marvelously exact transformation of the sound wave into electrical impulses or current. These impulses or this current is weak past understanding by any except the trained mind of an electrical engineer.
This current is passed through two stages of amplification. The process of amplification is described in full detail on pages 1025 to* 1035, to which I advise you to turn before continuing.
After the two-stage amplification above described, the current is sent to a mixing panel, located upon the main recorder amplifier, which same is for the purpose of combining the circuits from two or more microphones, if more than one is used.