small pad which rested with
frictional pressure on the periphery of the drum. This pad was carried
on the end of a vibrating arm whose lateral movement was limited between
two adjustable points. Normally, the frictional pressure between the
drum and pad would carry the latter with the former as it revolved, but
if the friction were removed a spring on the end of the vibrator arm
would draw it back to its starting-place.

In practice, the chalk drum was electrically connected with one pole of
an incoming telegraph circuit, and the vibrating arm and pad with the
other pole. When the drum rotated, the friction of the pad carried the
vibrating arm forward, but an electrical impulse coming over the line
would decompose the chemical solution with which the drum was moistened,
causing an effect similar to lubrication, and thus allowing the pad to
slip backward freely in response to the pull of its retractile spring.
The frictional movements of the pad with the drum were comparatively
long or short, and corresponded with the length of the impulses sent in
over the line. Thus, the transmission of Morse dots and dashes by the
distant operator resulted in movements of corresponding length by the
frictional pad and vibrating arm.

This brings us to the gist of the ingenious way in which Edison
substituted the action of electrochemical decomposition for that of the
electromagnet to operate a relay. The actual relaying was accomplished
through the medium of two contacts making connection with the local
or relay circuit. One of these contacts was fixed, while the other was
carried by the vibrating arm; and, as the latter made its forward and
backward movements, these contacts were alternately brought together or
separated, thus throwing in and out of circuit the battery and sounder
in the local circuit and causing a repetition of the incoming signals.
The other side of the local circuit was permanently connected to an
insulated block on the vibrator. This device not only worked with great
rapidity, but was extremely sensitive, and would respond to currents
too weak to affect the most delicate electromagnetic relay. It should
be stated that Edison did not confine himself to the working of the
electromotograph by the slipping of surfaces through the action of
incoming current, but by varying the character of the surfaces in
contact the frictional effect might be intensified by the electrical
current. In such a case the movements would