closed circuit is used on which is constantly
flowing a battery current, and included in that circuit is a
pair of electrodes, one or both of which is of carbon. These
electrodes are always in contact with a certain initial
pressure, so that current will be always flowing over the
circuit. One of the electrodes is connected with the
diaphragm on which the sound-waves impinge, and the
vibration of this diaphragm causes the pressure between the
electrodes to be correspondingly varied, and thereby effects
a variation in the current, resulting in the production of
impulses which actuate the receiving magnet. In other words,
with Bell's telephone the sound-waves themselves generate
the electric impulses, which are hence extremely faint. With
the Edison telephone, the sound-waves actuate an electric
valve, so to speak, and permit variations in a current of
any desired strength.

A second distinction between the two telephones is this:
With the Bell apparatus the very weak electric impulses
generated by the vibration of the transmitting diaphragm
pass over the entire line to the receiving end, and in
consequence the permissible length of line is limited to a
few miles under ideal conditions. With Edison's telephone
the battery current does not flow on the main line, but
passes through the primary circuit of an induction coil, by
which corresponding impulses of enormously higher potential
are sent out on the main line to the receiving end. In
consequence, the line may be hundreds of miles in length. No
modern telephone system in use to-day lacks these
characteristic features--the varying resistance and the
induction coil.]

The principle of the electromotograph was utilized by Edison in
more ways than one, first of all in telegraphy at this juncture. The
well-known Page patent, which had lingered in the Patent Office for
years, had just been issued, and was considered a formidable weapon. It
related to the use of a retractile spring to withdraw the armature
lever from the magnet of a telegraph or other relay or sounder, and thus
controlled the art of telegraphy, except in simple circuits. "There was
no known way," remarks Edison, "whereby this patent could be evaded, and
its possessor would eventually control the use of what is known as the
relay and sounder, and this was vital