which are volatilized by the current jumping from one to the other. A magnet, whirled outside the bell-glass, draws round the iron armature on the pivoted table, and consequently revolves the record, on the surface of which a very thin coating of gold is deposited. The record is next placed in an electroplating bath until a copper shell one-sixteenth of an inch thick has formed all over the outside. This is trued up on a lathe and encased in a brass tube. The "master," or original wax record, is removed by cooling it till it contracts sufficiently to fall out of the copper mould, on the inside surface of which are reproduced, in relief, the indentations of the wax "master."
Copies are made from the mould by immersing it in a tank of melted wax. The cold metal chills the wax that touches it, so that the mould soon has a thick waxen lining. The mould and copy are removed from the tank and mounted on a lathe, which shapes and smooths the inside of the record. The record is loosened from the mould by cooling. After inspection for flaws, it is, if found satisfactory, packed in cotton-wool and added to the saleable stock.
Gramophone master records are made on a circular disc of zinc, coated over with a very thin film of acid-proof fat. When the disc is revolved in the recording machine, the sharp stylus cuts through the fat and exposes the zinc beneath. On immersion in a bath of chromic acid the bared surfaces are bitten into, while the unexposed parts remain unaffected. When the etching is considered complete, the plate is carefully cleaned and tested. A negative copper copy is made from it by electrotyping. This constitutes the mould. From it as many as