rength 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