inexplicable; namely, the non-action of the same metals and magnets when at
rest. These results, which also afford the readiest means of obtaining
electricity from magnetism, I shall now proceed to describe.


§ 4. _Explication of Arago's Magnetic Phenomena._


81. If a plate of copper be revolved close to a magnetic needle, or 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. Ampère,