ough one wire did, in reality,
induce a similar current through the other wire, but that it continued
for an instant only, and partook more of the nature of the electrical
wave passed through from the shock of a common Leyden jar than of that
from a voltaic battery, and, therefore, might magnetize a steel needle
although it scarcely affected the galvanometer.

"This expectation was confirmed; for on substituting a small hollow
helix, formed round a glass tube, for the galvanometer, introducing
a steel needle, making contact as before between the battery and the
inducing wire, and then removing the needle before the battery contact
was broken, it was found magnetized.

"When the battery contact was first made, then an unmagnetized needle
introduced, and lastly the battery contact broken, the needle was found
magnetized to an equal degree apparently with the first; but the poles
were of the contrary kinds."(3)

To Faraday these experiments explained the phenomenon of Arago's
rotating disk, the disk inducing the current from the magnet, and, in
reacting, deflecting the needle. To prove this, he constructed a disk
that revolved between the poles of an electro-magnet, connecting the
axis and the edge of the disk with a galvanometer. "... A disk of
copper, twelve inches in diameter, fixed upon a brass axis," he says,
"was mounted in frames so as to be revolved either vertically or
horizontally, its edge being at the same time introduced more or less
between the magnetic poles. The edge of the plate was well amalgamated
for the purpose of obtaining good but movable contact; a part round the
axis was also prepared in a similar manner.

"Conductors or collectors of copper and lead were constructed so as to
come in contact with the edge of the copper disk, or with other forms
of plates hereafter to be described. These conductors we're about four
inches long, one-third of an inch wide, and one-fifth of an inch thick;
one end of each was slightly grooved, to allow of more exact adaptation
to the somewhat convex edge of the plates, and then amalgamated. Copper
wires, one-sixteenth of an inch in thickness, attached in the ordinary
manner by convolutions to the other ends of these conductors, passed
away to the galvanometer.

"All these arrangements being made, the copper disk was adjusted, the
small magnetic poles being about one-half an inch apart, and the edge
of the plate inserted about half their width between them. One of t