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 both by Arago and
Ampere, and Poisson had published a theoretic memoir on the subject;
but no cause could be assigned for so extraordinary an action.
It had also been examined in this country by two celebrated men,
Mr. Babbage and Sir John Herschel; but it still remained a mystery.
Faraday always recommended the suspension of judgment in cases of
doubt. 'I have always admired,' he says, 'the prudence and
philosophical reserve shown by M. Arago in resisting the temptation
to give a theory of the effect he had discovered, so long as he
could not devise one which was perfect in its application, and in
refusing to assent to the imperfect theories of others.' Now,
however, the time for theory had come. Faraday saw mentally the
rotating disk, under the operation of the magnet, flooded with his
induced currents, and from the known laws of interaction between
currents and magnets he hoped to deduce the motion observed by
Arago. That hope he realised, showing by actual experiment that when
his disk rotated currents passed through it, their position and
direction being such as must, in accordance with the established
laws of electro-magnetic action, produce the observed rotation.

Introducing the edge of his disk between the poles of the large