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Page 29
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.
It has also been discovered more recently that in the operation of
electric railways in which the track rails form part of the circuit, a
considerable increase in the tractive adhesion of the driving wheels
is manifested, due to the passage of the return current from the
wheels into the track. In the Baltimore and Hampden electric railway,
using the Daft "third rail" system, this increased tractive adhesion
enables the motors to ascend without slipping a long grade of 350 feet
to the mile, drawing two heavily loaded cars, which result, it is
claimed, is not attainable by steam or other self-propelling motors of
similar weight. In the two instances just cited the conditions are
widely different, as regards the nature of the current employed, the
mechanical properties of the surfaces in contact, and the electrical
resistance and the working conditions of the respective circuits. In
both, however, as clearly demonstrated by the experiments hereinafter
referred to, the cause of the increased friction is substantially the
same.
In order to ascertain the practical value of the electric current as a
means of increasing mechanical friction, and, if possible, render it
commercially and practically useful wherever such additional friction
might be desirable, as for example in the transmission of power, etc.,
a series of experiments were entered into by the author, which, though
not yet fully completed, are sufficiently advanced to show that an
electric current, when properly applied, is capable of very materially
increasing the mechanical friction of rotating bodies, in some cases
as much as from 50 to 100 per cent., with a very economical
expenditure of current; this increase depending upon the nature of the
substances in contact and being capable of being raised by an
increased flow of current.
Before entering into a description of the means by which this result
is produced, and how it is proposed to apply this method practically
to railway and other purposes, it may be well to give a general
outline of what has so far been determined. These experiments have
shown that the coefficient of friction between two conducting surfaces
is very much increased by the passage therethrough of an electric
current of _low electromotive force and large volume_, and this is
especially noticeable between two rolling surfaces in peripheral
contact with each other, or between a rolling and a stationary
surface, as in the case of a driving wheel running upon a railway
rail. This effect increases with the number of amperes of current
flowing through the circuit, of which the two surfaces form part, and
is not materially affected by the electromotive force, so long as the
latter is sufficient to overcome the electrical resistance of the
circuit. This increase in frictional adhesion is principally
noticeable in iron, steel, and other metallic bodies, and is due to a
molecular change in the conducting substances at their point of
contact (which is also the point of greatest resistance in the
circuit), caused by the heat developed at that point. This heat is
ordinarily imperceptible, and becomes apparent only when the current
strength is largely augmented. It is therefore probable that a portion
of this increased tractive adhesion is due directly to the current
itself aside from its heating effect, although I have not as yet been
able to ascertain this definitely. The most economical and efficient
results have been obtained by the employment of a transformed current
of extremely low electromotive force (between � and 1 volt), but of
very large volume or quantity, this latter being variable at will, so
as to obtain different degrees of frictional resistance in the
substances under observation.
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