I say "comparatively new" because the underlying principles involved
in the experiments referred to have, to a certain extent, been
employed (in, however, a somewhat restricted sense) for purposes
analogous to those that form the basis of this communication.
As indicated by the title, the subject that will now occupy our
attention is the use of the electric current as a means of increasing
and varying the frictional adhesion of rolling contacts and other
rubbing surfaces, and it is proposed to show how this effect may be
produced, both by means of the direct action of the current itself and
by its indirect action through the agency of electro-magnetism.
Probably the first instance in which the electric current was directly
employed to vary the amount of friction between two rubbing surfaces
was exemplified in Edison's electro-motograph, in which the variations
in the strength of a telephonic current caused corresponding
variations in friction between a revolving cylinder of moistened chalk
and the free end of an adjustable contact arm whose opposite extremity
was attached to the diaphragm of the receiving telephone. This device
was extremely sensitive to the least changes in current strength, and
if it were not for the complication introduced by the revolving
cylinder, it is very likely that it would to-day be more generally
used.
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