Fig. 146.—The "governor" of a phonograph.
We do not propose to trace here the development of the talking-machine; nor will it be necessary to describe in detail its mechanism, which is probably well known to most readers, or could be mastered in a very short time on personal examination. We will content ourselves with saying that the wax cylinder of the phonograph, or the ebonite disc of the gramophone, is generally rotated by clockwork concealed in the body of the machine. The speed of rotation has to be very carefully governed, in order that the record may revolve under the reproducing point at a uniform speed. The principle of the governor commonly used appears in Fig. 146. The last pinion of the clockwork train is mounted on a shaft carrying two triangular plates, A and C, to which are attached three short lengths of flat steel spring with a heavy ball attached to the centre of each. A is fixed; C moves up the shaft as the balls fly out, and pulls with it the disc D, which rubs against the pad P (on the end of a spring) and sets up sufficient friction to slow the clockwork. The limit rate is regulated by screw S.
THE PHONOGRAPH.
Though the recording and reproducing apparatus of a phonograph gives very wonderful results, its construction is quite simple. At the same time, it must be borne in mind that an immense amount of experimenting has been devoted to finding out the most suitable materials and forms for the parts.
Fig. 147.—Section of an Edison Bell phonograph recorder.
The recorder (Fig. 147) is a little circular box about one and a half inches in diameter.[32] From the top a tube leads to the horn. The bottom is a circular plate, C C, hinged at one side. This plate supports a glass disc, D, about 1⁄150th of an inch thick, to which is attached the cutting stylus—a tiny sapphire rod with a cup-shaped end having very sharp edges. Sound-waves enter the box through the horn tube; but instead of being allowed to fill the whole box, they are concentrated by the shifting nozzle N on to the centre of the glass disc through the hole in C C. You will notice that N has a ball end, and C C a socket to fit N exactly, so that, though C C and N move up and down very rapidly, they still make perfect contact. The disc is vibrated by the sound-impulses, and drives the cutting point down into the surface of the wax cylinder, turning below it in a clockwork direction. The only dead weight pressing on S is that of N, C C, and the glass diaphragm.
Fig. 148.—Perspective view of a phonograph recorder.