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RECORDING AND REPRODUCTION 423
mounted that it is free to vibrate in resonance with changes of air pressure carried to it by a horn or megaphone. Consequently, when sound (air) vibrations impinge upon this disc it will bend inward and bulge outward in resonance with the frequency of the sound, precisely as does the human eardrum, and for precisely the same reasons.
(3) A metal needle or stylus is rigidly attached to the center of the disc, so that every vibration the disc undergoes must be duplicated by the point of the needle. The needle may be mounted above a turn-table and on this turn-table may be placed a very thick, blank phonograph record made of soft wax. The turn-table is made to rotate at some fixed, unvarying speed, the needle is lowered until it cuts into the moving wax. Since the needle, together with its disc or diaphragm, is mounted upon a worm gear which moves it slightly sidewise at each rotation of the wax, it cuts a spiral groove, as may be seen on any phonograph record.
Now whenever sound waves reach the little metal diaphragm the point of the stylus must vibrate in resonance with them. In most recording this vibration causes the groove cut by the stylus to waver slightly from side to side. (4) Some records are so made that the vibrating point cuts deeper or shallower, creating what is known as "vertical cut" or "hiU and dale" recording. The fundamental principle of "hill and dale" is, incidentally, not new. It is older than the other, or lateral, cut, but has recently been iiuproved to give strikingly superior results. Whichever method is used the wax is engraved with a series of physical markings that corresponds exactly to the sequence of changes in air pressure. The slight variations in the groove cut by the stylus are the work the sound performed while it lasted, and by means of them a duplicate of that sound can be created at any time after the record has been made permanent. This is done by dusting the wax writh carbon powder and electroplating metal upon it.
Assume that the wax has been so treated, and the resultant metal perhaps used to manufacture many simi