, 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 1/2 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.

These experiments were originally directed mainly toward an endeavor
to increase the tractive adhesion of the driving wheels of locomotives
and other vehicles, and to utilize the electric current for this
purpose in such a manner as to render it entirely safe, practical, and
economical. It will be apparent at once that a method of increasing
the tractive power of the present steam locomotives by more than 50
per cent. without adding to their weight and without injury to the
roadbed and wheel tires, such as is caused by the sand now commonly
used, would prove of considerable value, and the same holds true with
respect to electrically propelled street cars, especially as it has
been found exceedingly difficu