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 attraction as a result.

[Illustration: Fig. 3]

The experiment was, however, varied, as in Fig. 1. The disk, D, was held
over the magnet pole, as shown, and the current in the magnet coils cut
off by shunting them. There was felt an attraction of the disk or a dip
toward the pole. The current was then put on by opening the shunting
switch, and a repulsive action or lift of the disk was felt. The actions
just described are what would be expected in such a case, for when
attraction took place, currents had been induced in the disk, D, in the