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526 RICHARDSON'S BLUEBOOK OF PROJECTION
The disc reproducer needs comparatively little attention, but the record and its turntable, and the reproducer mounting, or tone-arm, are almost as delicate in their requirements as the photo-electric cell equipment. Both turntable and tone-arm must be perfectly level, or the needle is likely to leave one groove and jump (fall downward) into another. A spirit level is used to check them from time to time. The record must turn smoothly, without any jerkiness in its motion, or flutter will result.
The needle point must be sharp enough to track through those slight variations of the groove that represent the highest frequencies of sound engraved on the record.
"Crystal" type pick-ups have come into the market very recently. They do not contain a magnet and a coil of wire, but a thin crystal of rochelle salts. A crystal of that salt will, for reasons unknown, generate current when it is subjected to a twisting motion. The needle and needle-holders are arranged to impart such a motion to the crystal in response to the irregularities in the groove of a moving record.
(31) "Hill and dale" records are those in which the groove is cut deeper and shallower, instead of wavering from side to side. In the common, or lateral-cut records, the spacing between grooves limits the amount of wavering possible, and therefore the volume. There is no such limit in the hill and dale. Hill and dale reproducers of course differ in construction from lateral-cut reproducers, in that the needle vibration must be up-and-down instead of from side to side.
Sound-on-disc can be drawn from any phonograph equipped with an electrical reproducer for exit marches or incidental music. A few years ago sound-on-disc records were furnished with film by the exchanges. These records were played on a turntable driven through an extension shaft by the same motor that operated the projector, hence were in synchronism with the picture. That method of securing synchronized sound is now obsolete in theatre work, but the old synchronous turntables are still used for incidental music in a few houses.