speed of
rotation is not sufficiently uniform to secure alone efficient
regulation of electromotive force. Another device, patented by Mr.
Crompton, is a modification of that method of field magnet winding
commonly known as compound winding. The field magnets are wound over
with two wires, one of which has a high resistance and is arranged as
a shunt, and the other of which has a low resistance and is arranged
in series. Instead, however, of the magnetizing powers of these coils
being united in the same direction as an ordinary compound winding,
they are opposed to one another. That is to say, the current in the
shunt wire tends to magnetize the iron of the field magnets in an
opposite direction to that of the series wire. It results from this
that any slight increase of speed diminishes the strength of the
magnetic field, and _vice versa_. Accordingly, within certain limits,
the electromotive force of the dynamo is independent of the speed of
rotation.

* * * * *

THE ELECTRIC CURRENT AS A MEANS OF INCREASING THE TRACTIVE ADHESION OF
RAILWAY MOTORS AND OTHER ROLLING CONTACTS.[1]

[Footnote 1: Read before the American Association for the
Advancement of Science. New York meeting, 1887.]

By ELIAS E. RIES.

The object of this paper is to lay before you the results of some
recent experiments in a comparatively new field of operation, but one
that, judging from the results already attained, is destined to become
of great importance and value in its practical application to various
branches of industry.

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 st