magnet,
suspended in such a way that the latter may rotate in a plane parallel to
that of the former, the magnet tends to follow the motion of the plate; or
if the magnet be revolved, the plate tends to follow its motion; and the
effect is so powerful, that magnets or plates of many pounds weight may be
thus carried round. If the magnet and plate be at rest relative to each
other, not the slightest effect, attractive or repulsive, or of any kind,
can be observed between them (62.). This is the phenomenon discovered by M.
Arago; and he states that the effect takes place not only with all metals,
but with solids, liquids, and even gases, i.e. with all substances (130.).

82. Mr. Babbage and Sir John Herschel, on conjointly repeating the
experiments in this country[A], could obtain the effects only with the
metals, and with carbon in a peculiar state (from gas retorts), i.e. only
with excellent conductors of electricity. They refer the effect to
magnetism induced in the plate by the magnet; the pole of the latter
causing an opposite pole in the nearest part of the plate, and round this a
more diffuse polarity of its own kind (120.). The essential circumstance in
producing the rotation of the suspended magnet is, that the substance
revolving below it shall acquire and lose its magnetism in sensible time,
and not instantly (124.). This theory refers the effect to an attractive
force, and is not agreed to by the discoverer, M. Arago, nor by M. Ampere,
who quote against it the absence of all attraction when the magnet and
metal are at rest (62. 126.), although the induced magnetism should still
remain; and who, from experiments made with a long dipping needle, conceive
the action to be always repulsive (125.).

[A] Philosophical Transactions, 1825, p. 467.

83. Upon obtaining electricity from magnets by the means already described
(36 46.), I hoped to make the experiment of M. Arago a new source of
electricity; and did not despair, by reference to terrestrial
magneto-electric induction, of being able to construct a new electrical
machine. Thus stimulated, numerous experiments were made with the magnet of
the Royal Society at Mr. Christie's house, in all of which I had the
advantage of his assistance. As many of these were in the course of the
superseded by more perfect arrangements, I shall consider myself at liberty
investigation to rearrange them in a manner calculated to convey most
readily what appears to me to be a cor