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988 HANDBOOK OF PROJECTION FOR
the groove caused by the recording needle vibrating up and down, thus making a series of indentations in the floor of the groove formed by it, which same constitute the sound record. This, however, is not the case here.
Examining a phonograph record under a fairly powerful glass it will be found that the sides of the grooves made by recording needle are not straight. Of course they curve, but that is not what I refer to here. Their sides instead of being a "straight" curve side, wobble or "wiggle" from side to side slightly, and these side-toside movements constitute the sound record. They are in fact a true profile of the recorded sound wave or vibration.
When you place the needle in a record and start the projector, as the record revolves the needle point, following these undulations in the groove sides, moves slightly from side to side throughout the entire run of the record, and it is this movement which, as will be explained under "Reproduction," page 1109, reproduces the sound. But that is getting ahead of our story.
The record is made as follows : As has been explained, the current reaches the recording apparatus from the main amplifier, just as it reaches the Aeo lamp in film recording. In this case, however, instead of an Aeo lamp fluctuating in brilliance with the sound waves, the phonograph recording needle is, by suitable means not necessary to describe here, made to oscillate from side to side in synchronism with the sound wave frequency. For the method of reproducing the record sound see page 1109.