tromotive force will, if plotted, be a wave
line, the amplitude of which is the arithmetical sum of the positive and
negative maxima of current or electromotive force, as the case may be,
while a horizontal middle line joins the zero points of current or
electromotive force.

[Illustration: FIG. 1]

It is well known that such a current passing in a coil or conductor laid
parallel with or in inductive relation to a second coil or conductor,
will induce in the second conductor, if on open circuit, alternating
electromotive forces, and that if its terminals be closed or joined,
alternating currents of the same rhythm, period, or pitch, will
circulate in the second conductor. This is the action occurring in any
induction coil whose primary wire is traversed by alternating currents,
and whose secondary wire is closed either upon itself directly or
through a resistance. What I desire to draw attention to in the present
paper are the mechanical actions of attraction and repulsion which will
be exhibited between the two conductors, and the novel results which may
be obtained by modifications in the relative dispositions of the two
conductors.

[Illustration: FIG. 2.]

In 1884, while preparing for the International Electrical Exhibition at
Philadelphia, we had occasion to construct a large electro-magnet, the
cores of which were about six inches in diameter and about twenty inches
long. They were made of bundles of iron rod of about 5/16 inch diameter.
When complete, the magnet was energized by the current of a dynamo
giving continuous currents, and it exhibited the usual powerful magnetic
effects. It was found also that a disk of sheet copper, of about 1/16
inch thickness and 10 inches in diameter, if dropped flat against a pole
of the magnet, would settle down softly upon it, being retarded by the
development of currents in the disk due to its movement in a strong
magnetic field, and which currents were of opposite direction to those
in the coils of the magnet. In fact, it was impossible to strike the
magnet pole a sharp blow with the disk, even when the attempt was made
by holding one edge of the disk in the hand and bringing it down
forcibly toward the magnet. In attempting to raise the disk quickly off
the pole, a similar but opposite action of resistance to movement took
place, showing the development of currents in the same direction to
those in the coils of the magnet, and which currents, of course, would
cause attr