same property might be applied to regulate the
strength of a current in conformity with the vibrations of the
voice, and after a great number of experiments produced his 'carbon
transmitter.' Plumbago in powder, in sticks, or rubbed on fibres and
sheets of silk, were tried as the sensitive material, but finally
abandoned in favour of a small cake or wafer of compressed lamp-black,
obtained from the smoke of burning oil, such as benzolene or rigolene.
This was the celebrated 'carbon button,' which on being placed between
two platinum discs by way of contact, and traversed by the electric
current, was found to vary in resistance under the pressure of the sound
waves. The voice was concentrated upon it by means of a mouthpiece and a
diaphragm.

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. He attached the pin or stylus to the centre of the mica, and
brought its point to bear on a cylindrical surface of prepared chalk.
The undulatory current from the line was passed through the stylus and
the chalk, while the latter was moved by turning a handle; and at every
pulse of the electricity the friction between the pin and chalk was
diminished, so that the stylus slipped upon its surface. The consequence
was a vib