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 end is left
free to vibrate over the other pole of the magnet and not quite touching
it. Each of the reeds is to be tuned to a different pitch, say the 1, 3,
5, and 8 of the scale. These electro-magnets with their attached
vibrators are to be attached each to a resonant box (see p. 93), which
can respond to that particular number of vibrations per second. This is
the receiving instrument. The sender consists of a like set of reeds
tuned to the same pitch, which can be made to vibrate at will by
pressing a key which sends the current of electricity through its
electro-magnet, which makes and breaks the current. Imagine one of these
keys to be pressed down so as to make the circuit complete: the sending
instrument then has one of its reeds, let it be the 1 of the scale, set
in vibration; the intermittent current traverses the whole line, going
through all four of the receiving instruments. Now, we know from the
study of the action of sounding bodies, that only one of the four
receivers is competent to vibrate in consonance with this tone, and this
one will respond; that is, the vibrations are truly sympathetic
vibrations. If, instead of making the 1 of the scale in the
sending-instrument, the 3 had been made, the current would have gone
through all of the receiving instruments just the same as before, but
only one of them could take up that vibratory movement: three of them
would remain at rest, the 3 responding loudly. In like manner, any
number of vibrating reeds in the sending instrument can make a
corresponding number of reeds in the receiving instrument to vibrate,
provided the latter be exactly tuned with the former. Each transmitter
is connected with but a part of the battery, so that several tones may
be transmitted at the same time. If the performer plays a piece of music
in its various parts, every part will be reproduced: thus we have a
compound or multiple telephone. This instrument has been used during
the past winter to give concerts in cities when the performer was in a
distant place.
It has also been used as a multiple telegraph; as many as eight
operators sending messages simultaneously over the same wire,--four in
each direction,--without the slightest interference.