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18 AMATEUR TALKING PICTURES
the sake of economy we generally use the same apparatus for recording as for reproducing. Even in gramophone recording studios a recording machine is sometimes employed for "playing back" as a check on performance.
It is with the medium of recording, the record or disc itself, that difference is encountered. To explain further we must trace briefly the story of the modern gramophone record.
As mentioned before, professional recording takes place on wax discs, which are shaved to a super-fine finish in special machines. After the wax has been recorded it is plated, sometimes with copper and sometimes with gold. To render plating possible the wax surface is first made electrically conductive by brushing with fine graphite powder. Alternatively the wax is plated by bombardment in a vacuum chamber after the style of the electronic action in a radio valve. The thin plating is then backed by further deposition, and the shell is then stripped from the wax and becomes the "Master" negative in which the originally recorded sound grooves appear as raised lines. Obviously this master could be employed for pressing records similar in form to the original wax.
Record pressing is very simple. A mould is fixed in the jaws of a steam -heated press, and upon it is placed a lump of moulding compound, usually a mixture of shellac, copal, and resin as bonding agents, with slate powder and carbon black as fillers. The press doors are then closed by hydraulic power exerting a pressure of about two tons per square inch, and in less than a minute heat and pressure have done their work, and a modern miracle is wrought in the form of a gramophone record. The steam is cut off and the mould chilled by water when the finished record is removed.
For commercial pressing of records the masters are not employed. From them by another electro -plating process is made a "Mother" shell, and from this a "matrix" This mother gives us a copy of the master which itself is kept for further reproductions, and is not employed in pressing. The development of the process, although very complicated, has been brought to such a fine art that practically no quality is lost in transference.