The property on which the receiver was based had been observed and
applied by him some time before. When a current is passed from a metal
contact through certain chemical salts, a lubricating effect was
noticeable. Thus if a metal stylus were rubbed or drawn over a prepared
surface, the point of the stylus was found to slip or 'skid' every time
a current passed between them, as though it had been oiled. If your pen
were the stylus, and the paper on which you write the surface, each wave
of electricity passing from the nib to the paper would make the pen
start, and jerk your fingers with it. He applied the property to the
recording of telegraph signals without the help of an electro-magnet, by
causing the currents to alter the friction between the two rubbing
surfaces, and so actuate a marker, which registered the message as in
the Morse system.
This instrument was called the 'electromotograph,' and it occurred to
Edison that in a similar way the undulatory currents from his carbon
transmitter might, by varying the friction between a metal stylus and
the prepared surface, put a tympanum in vibration, and reproduce the
original sounds. Wonderful as it may appear, he succeeded in doing so
by the aid of a piece of chalk, a brass pin, and a thin sheet or disc of
mica.
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