sc 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.

[Illustration: 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.

[Illustration: FIG. 148.--Perspective view of a phonograph recorder.]

As the cylinder revolves, the recorder is shifted continuously along by
a leading screw having one hundred or more threads to the inch cut on
it, so that it traces a continuous helical groove from one end of the
wax cylinder to the other. This groove is really a s