s the conducting
power of the substance; and I venture now to predict that such will be
found to be the case; and that in all those instances where non-conductors
have been supposed to exhibit this peculiar influence, the motion has been
due to some interfering cause of an ordinary kind; as mechanical
communication of motion through the parts of the apparatus, or otherwise
(as in the case Mr. Harris has pointed out[A]); or else to ordinary
magnetic attractions. To distinguish the effects of the latter from those
of the induced electric currents, I have been able to devise a most perfect
test, which shall be almost immediately described (243.).

[A] Philosophical Transactions, 1831. p. 68.

216. There is every reason to believe that the magnet or magnetic needle
will become an excellent measurer of the conducting power of substances
rotated near it; for I have found by careful experiment, that when a
constant current of electricity was sent successively through a series of
wires of copper, platina, zinc, silver, lead, and tin, drawn to the same
diameter; the deflection of the needle was exactly equal by them all. It
must be remembered that when bodies are rotated in a horizontal plane, the
magnetism of the earth is active upon them. As the effect is general to the
whole of the plate, it may not interfere in these cases; but in some
experiments and calculations may be of important consequence.

217. Another point which I endeavoured to ascertain, was, whether it was
essential or not that the moving part of the wire should, in cutting the
magnetic curves, pass into positions of greater or lesser magnetic force;
or whether, always intersecting curves of equal magnetic intensity, the
mere motion was sufficient for the production of the current. That the
latter is true, has been proved already in several of the experiments on
terrestrial magneto-electric induction. Thus the electricity evolved from
the copper plate (149.), the currents produced in the rotating globe (161,
&c.), and those passing through the moving wire (171.), are all produced
under circumstances in which the magnetic force could not but be the same
during the whole experiments.

218. To prove the point with an ordinary magnet, a copper disc was cemented
upon the end of a cylinder magnet, with paper intervening; the magnet and
disc were rotated together, and collectors (attached to the galvanometer)
brought in contact with the circumference and the centra