the bobbin excites corresponding waves of electricity in the coil,
after the famous experiment of Faraday (page 64). If this
undulatory current is passed through the coil of a similar
telephone at the far end of the line, it will, by a reverse
action, set the diaphragm in vibration and reproduce the original
sonorous waves. The result is, that when another person listens at
the mouthpiece of the receiving telephone, he will hear a faithful
imitation of the original speech.

The Bell telephone is virtually a small magneto-electric generator
of electricity, and when two are joined in circuit we have a
system for the transmission of energy. As the voice is the motive
power, its talk, though distinct, is comparatively feeble, and
further improvements were made before the telephone became as
serviceable as it is now.

Edison, in 1877, was the first to invent a working telephone,
which, instead of generating the current, merely controlled the
strength of it, as the sluice of a mill-dam regulates the flow of
water in the lead. Du Moncel had observed that powder of carbon
altered in electrical resistance under pressure, and Edison found
that lamp-black was so sensitive as to change in resistance under
the impact of the sonorous waves. His transmitter consisted of a
button or wafer of lamp-black behind a diaphragm, and connected in
the circuit. On speaking to the diaphragm the sonorous waves
pressed it against the button, and so varied the strength of the
current in a sympathetic manner. The receiver of Edison was
equally ingenious, and consisted of a cylinder of prepared chalk
kept in rotation and a brass stylus rubbing on it. When the
undulatory current passed from the stylus to the chalk, the stylus