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998 HANDBOOK OF PROJECTION FOR
From the mixing panel the current passes directly to the main amplifier panel and passes directly into the first stage of amplification of the main amplifier, and thence through four successive stages of amplification before being finally sent forward to the apparatus it is to operate.
Some idea of the weakness of the microphone current may be had when I tell you that before reaching the point of use at the loud speakers it has been amplified at least one hundred million times. No one seems certain as to the exact figure. Some engineers have placed it as high as half a billion times, so I think 100,000,000, huge as it seems, is probably not excessive.
From its last stage of amplification the current is sent directly to the sound recorder oscillograph galvanometer, which it is to operate, and in so doing record the sound upon the film in the manner I shall now describe.
HOW THE RECORD IS MADE.— The oscillograph galvanometer consists of a molybdenum loop, through which the amplified microphone current circulates. This loop is suspended in a magnetic field, and to it a small mirror is cemented.
The light for recording the sound upon the film sound track is supplied by a small incandescent bulb, very similar in external appearance to those used in automobile headlights. It, however, has a coiled filament forming a straight line. This filament is suspended or held in horizontal position. Light from it is passed through a small condenser lens and focused upon the beforenamed mirror, from which it is reflected through another condensing lens and concentrated upon a slit formed by two microscopically adjusted knife edges, so