In this apparatus a vibrating
steel tongue interrupted the current, which at the other end of the line
passed through the electro-magnet and vibrated a band or tongue of iron
near its poles. Gray's 'harmonic telegraph,' with the vibrating tongues
or reeds, was afterwards introduced on the lines of the Western Union
Telegraph Company in America. As more than one set of vibrations--that
is to say, more than one note--can be sent over the same wire
simultaneously, it is utilised as a 'multiplex' or many-ply telegraph,
conveying several messages through the same wire at once; and these can
either be interpreted by the sound, or the marks drawn on a ribbon of
travelling paper by a Morse recorder.
Gray also invented a 'physiological receiver,' which has a curious
history. Early in 1874 his nephew was playing with a small induction
coil, and, having connected one end of the secondary circuit to the zinc
lining of a bath, which was dry, he was holding the other end in his
left hand. While he rubbed the zinc with his right hand Gray noticed
that a sound proceeded from it, which had the pitch and quality of the
note emitted by the vibrating contact or electrotome of the coil. 'I
immediately took the electrode in my hand,' he writes, 'and, repeating
the operation, found to my astonishment that by rubbing hard and rapidly
I could make a much louder sound than the electrotome.
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