sk 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
horseshoe magnet of the Royal Society, and connecting the axis and the
edge of the disk, each by a wire with a galvanometer, he obtained, when
the disk was turned round, a constant flow of electricity. The direction
of the current was determined by the direction of the motion, the
current being reversed when the rotation was reversed. He now states the
law which rules the production of currents in both disks and wires, and
in so doing uses, for the first time, a phrase which has since become
famous. When iron filings are scattered over a magnet, the particles
of iron arrange themselves in certain determinate lines called magnetic
curves. In 1831, Faraday for the first time called these curves 'lines
of magnetic force'; and he showed that to produce induced currents
neither approach to nor withdrawal from a magnetic source, or centre, or
pole, was essential, but that it was only necessary to cut appropriately
the lines of magnetic force. Faraday's first paper on Magneto-electric
Induction, which I have here endeavoured to condense, was read before