anometer, he found that the moment the ring was
magnetized, by sending a current through _the other coil_, the
galvanometer needle whirled round four or five times in succession. The
action, as before, was that of a pulse, which vanished immediately. On
interrupting the current, a whirl of the needle in the opposite
direction occurred. It was only during the time of magnetization or
demagnetization that these effects were produced. The induced currents
declared a _change_ of condition only, and they vanished the moment the
act of magnetization or demagnetization was complete.

The effects obtained with the welded ring were also obtained with
straight bars of iron. Whether the bars were magnetized by the electric
current, or were excited by the contact of permanent steel magnets,
induced currents were always generated during the rise, and during the
subsidence of the magnetism. The use of iron was then abandoned, and the
same effects were obtained by merely thrusting a permanent steel magnet
into a coil of wire. A rush of electricity through the coil accompanied
the insertion of the magnet; an equal rush in the opposite direction
accompanied its withdrawal. The precision with which Faraday describes
these results, and the completeness with which he defined the boundaries
of his facts, are wonderful. The magnet, for example, must not be passed
quite through the coil, but only half through, for if passed wholly
through, the needle is stopped as by a blow, and then he shows how this
blow results from a reversal of the electric wave in the helix. He next
operated with the powerful permanent magnet of the Royal Society, and
obtained with it, in an exalted degree, all the foregoing phenomena.

And now he turned the light of these discoveries upon the darkest
physical phenomenon of that day. Arago had discovered in 1824, that a
disk of non-magnetic metal had the power of bringing a vibrating
magnetic needle suspended over it rapidly to rest; and that on causing
the disk to rotate the magnetic needle rotated along with it. When both
were quiescent, there was not the slightest measurable attraction or
repulsion exerted between the needle and the disk; still when in motion
the disk was competent to drag after it, not only a light needle, but a
heavy magnet. The question had been probed and investigated with
admirable skill by both Arago and Ampere, and Poisson had published a
theoretic memoir on the subject; but no cause could be