of this instrument.

Dr. Wright found that a sound of considerable intensity could be
obtained by passing the interrupted current through the primary wire of
a small induction coil, and placing a conductor made of two sheets of
silvered paper placed back to back in the secondary circuit. The
silvered paper becomes rapidly charged and discharged, making a sound
that can be heard over a large hall, and having the same pitch as the
sending instrument.

GRAY'S TELEPHONES.

In 1873 Mr. Elisha Gray of Chicago discovered that if an induction coil
be made to operate by the current from any automatic circuit-breaker,
and one of the wires from the secondary circuit be held in the hand
while the dry finger of the same hand is rubbed upon a sonorous metallic
plate, the other wire being in connection with the plate, a musical
sound would be given out by the plate, appearing to come from the point
of contact of the finger with the plate. He therefore contrived a
musical instrument with a range of two octaves, in which the reeds were
made to vibrate by electro magnets, the current entering any one by
depressing the appropriate key. This circuit is sent through the primary
wire of an induction coil while one of the terminals of the secondary
coil is connected with the thin sheet metal that forms one head of a
shallow wooden drum about eight inches in diameter, which is so fixed as
to be rotated like a pulley. The other terminal is held in the hand
while one finger of the same hand rests upon the metallic surface. While
the drum is turned with the other hand, the sounds given out have
considerable intensity. The faster the drum is turned, the louder do the
sounds become, though the pitch remains the same.

In this case, as in the case mentioned on p. 105, we have an electric
current passing between two surfaces that are moving upon each other;
the contact not being uniform, the current is varying as well as
intermittent.

Mr. Gray has also invented a musical telephone by means of which many
musical sounds may be simultaneously transmitted and reproduced. The
actual mechanism used is quite complex, and requires considerable
familiarity with electrical science in order to understand it; but the
fundamental principle involved is not difficult to one who has
comprehended the preceding descriptions.

Suppose that we have a series of four steel reeds, each one fixed at one
end to one pole of a short electro-magnet, while the other en