d with the metals by Arago, and those due to ordinary
magnetic forces; and henceforth, therefore, the application of two poles to
various moving substances will, if they appear at all magnetically
affected, afford a proof of the nature of that affection. If opposite poles
produce a greater effect than one pole, the result will be due to electric
currents. If similar poles produce more effect than one, then the power is
_not_ electrical; it is not like that active in the metals and carbon when
they are moving, and in most cases will probably be found to be not even
magnetical, but the result of irregular causes not anticipated and
consequently not guarded against.

255. The result of these investigations tends to show that there are really
but very few bodies that are magnetic in the manner of iron. I have often
sought for indications of this power in the common metals and other
substances; and once in illustration of Arago's objection (82.), and in
hopes of ascertaining the existence of currents in metals by the momentary
approach of a magnet, suspended a disc of copper by a single fibre of silk
in an excellent vacuum, and approximated powerful magnets on the outside of
the jar, making them approach and recede in unison with a pendulum that
vibrated as the disc would do: but no motion could be obtained; not merely,
no indication of ordinary magnetic powers, but none or _any electric
current_ occasioned in the metal by the approximation and recession of the
magnet. I therefore venture to arrange substances in three classes as
regards their relation to magnets; first, those which are affected when at
rest, like iron, nickel, &c., being such as possess ordinary magnetic
properties; then, those which are affected when in motion, being conductors
of electricity in which are produced electric currents by the inductive
force of the magnet; and, lastly, those which are perfectly indifferent to
the magnet, whether at rest or in motion.

256. Although it will require further research, and probably close
investigation, both experimental and mathematical, before the exact mode of
action between a magnet and metal moving relatively to each other is
ascertained; yet many of the results appear sufficiently clear and simple
to allow of expression in a somewhat general manner.--If a terminated wire
move so as to cut a magnetic curve, a power is called into action which
tends to urge an electric current through it; but this current cannot