the
voice control the intensity of a current which was independently
supplied to the line by a voltaic battery. The plan of Bell, in short,
may be compared to a man who employs his strength to pump a quantity
of water into a pipe, and that of Edison to one who uses his to open a
sluice, through which a stream of water flows from a capacious dam into
the pipe. Edison was acquainted with two experimental facts on which to
base the invention.

In 1873, or thereabout, he claimed to have observed, while constructing
rheostats, or electrical resistances for making an artificial telegraph
line, that powdered plumbago and carbon has the property of varying in
its resistance to the passage of the current when under pressure. The
variation seemed in a manner proportional to the pressure. As a matter
of fact, powdered carbon and plumbago had been used in making small
adjustable rheostats by M. Clerac, in France, and probably also in
Germany, as early as 1865 or 1866. Clerac's device consisted of a small
wooden tube containing the material, and fitted with contacts for the
current, which appear to have adjusted the pressure. Moreover, the
Count Du Moncel, as far back as 1856, had clearly discovered that when
powdered carbon was subjected to pressure, its electrical resistance
altered, and had made a number of experiments on the phenomenon. Edison
may have independently observed the fact, but it is certain he was not
the first, and his claim to priority has fallen to the ground.

Still he deserves the full credit of utilising it in ways which were
highly ingenious and bold. The 'pressure-relay,' produced in 1877, was
the first relay in which the strength of the local current working
the local telegraph instrument was caused to vary in proportion to
the variation; of the current in the main line. It consisted of an
electro-magnet with double poles and an armature which pressed upon a
disc or discs of plumbago, through which the local current Passed. The
electro-magnet was excited by the main line current and the armature
attracted to its poles at every signal, thus pressing on the plumbago,
and by reducing its resistance varying the current in the local circuit.
According as the main line current was strong or weak, the pressure
on the plumbago was more or less, and the current in the local circuit
strong or weak. Hence the signals of the local receiver were in
accordance with the currents in the main line.

Edison found that the