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IN solving a problem that at the time was
thought to be insurmountable, and in the
adaptability of its principles to the successful
overcoming of apparently insuperable difficulties
subsequently arising in other lines of work,
this invention is one of the most remarkable of
the many that Edison has made in his long career
as an inventor.
The object primarily sought to be accomplished
was the repeating of telegraphic signals from a
distance without the aid of a galvanometer or an
electromagnetic relay, to overcome the claims of
the Page patent referred to in the preceding
narrative. This object was achieved in the
device described in Edison's basic patent No.
158,787, issued January 19,
1875, by the substitution of friction and
anti- friction for the presence and absence of
magnetism in a regulation relay.
It may be observed, parenthetically, for the
benefit of the lay reader, that in telegraphy
the device known as the relay is a receiving
instrument containing an electromagnet adapted to
respond to the weak line-current. Its armature
moves in accordance with electrical impulses, or
signals, transmitted from a distance, and, in
so responding, operates mechanically to
alternately close and open a separate local
circuit in which there is a sounder and a
powerful battery. When used for true relaying
purposes the signals received from a distance are
in turn repeated over the next section of the
line, the powerful local battery furnishing
current for this purpose. As this causes a loud
repetition of the original signals, it will be
seen that relaying is an economic method of
extending a telegraph circuit beyond the natural
limits of its battery power.
At the time of Edison's invention, as related
in Chapter IX of the preceding narrative,
there existed no other known method than the one
just described for the repetition of transmitted
signals, thus limiting the application of
telegraphy to the pleasure of those who might own
any patent controlling the relay, except on
simple circuits where a single battery was
sufficient. Edison's previous discovery of
differential friction of surfaces through
electrochemical decomposition was now adapted by
him to produce motion at the end of a circuit
without the intervention of an electromagnet.
In other words, he invented a telegraph
instrument having a vibrator controlled by
electrochemical decomposition, to take the place
of a vibrating armature operated by an
electromagnet, and thus opened an entirely new
and unsuspected avenue in the art.
Edison's electromotograph comprised an
ingeniously arranged apparatus in which two
surfaces, normally in contact with each other,
were caused to alternately adhere by friction or
slip by reason of electrochemical decomposition.
One of these surfaces consisted of a small drum
or cylinder of chalk, which was kept in a
moistened condition with a suitable chemical
solution, and adapted to revolve continuously by
clockwork. The other surface consisted of a
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 be the
reverse of those above indicated, but the end
sought --namely, the relaying of
messages--would be attained with the same
certainty.
While the principal object of this invention was
to accomplish the repetition of signals without
the aid of an electromagnetic relay, the
instrument devised by Edison was capable of use
as a recorder also, by employing a small wheel
inked by a fountain wheel and attached to the
vibrating arm through suitable mechanism. By
means of this adjunct the dashes and dots of the
transmitted impulses could be recorded upon a
paper ribbon passing continuously over the drum.
The electromotograph is shown diagrammatically
in Figs. 1 and 2, in plan and vertical
section respectively. The reference letters in
each case indicate identical parts: A being the
chalk drum, B the paper tape, C the auxiliary
cylinder, D the vibrating arm, E the
frictional pad, F the spring, G and H the
two contacts, I and J the two wires leading to
local circuit, K a battery, and L an ordinary
telegraph key. The two last named, K and L,
are shown to make the sketch complete but in
practice would be at the transmitting end, which
might be hundreds of miles away. It will be
understood, of course, that the
electromotograph is a receiving and relaying
instrument.
Another notable use of the electromotograph
principle was in its adaptation to the receiver
in Edison's loud-speaking telephone, on which
United States Patent No. 221,957 was
issued November 25, 1879. A chalk
cylinder moistened with a chemical solution was
revolved by hand or a small motor. Resting on
the cylinder was a palladium-faced pen or
spring, which was attached to a mica diaphragm
in a resonator. The current passed from the
main line through the pen to the chalk and to the
battery. The sound-waves impinging upon the
distant transmitter varied the resistance of the
carbon button therein, thus causing
corresponding variations in the strength of the
battery current. These variations, passing
through the chalk cylinder produced more or less
electrochemical decomposition, which in turn
caused differences of adhesion between the pen
and cylinder and hence gave rise to mechanical
vibrations of the diaphragm by reason of which
the speaker's words were reproduced.
Telephones so operated repeated speaking and
singing in very loud tones. In one instance,
spoken words and the singing of songs originating
at a distance were heard perfectly by an audience
of over five thousand people.
The loud-speaking telephone is shown in
section, diagrammatically, in the sketch
(Fig. 3), in which A is the chalk cylinder
mounted on a shaft, B. The palladium-faced
pen or spring, C, is connected to diaphragm
D. The instrument in its commercial form is
shown in Fig. 4.
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