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.
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