ion to the fixed wire under induction as if _it_ had moved in the
opposite direction across them, or towards the wire carrying the current.
Hence the first current induced in such cases was in the contrary direction
to the principal current (17. 235.). On breaking the battery contact, the
magnetic curves (which are mere expressions for arranged magnetic forces)
may be conceived as contracting upon and returning towards the failing
electrical current, and therefore move in the opposite direction across the
wire, and cause an opposite induced current to the first.

239. When, in experiments with ordinary magnets, the latter, in place of
being moved past the wires, were actually made near them (27. 36.), then a
similar progressive development of the magnetic curves may be considered as
having taken place, producing the effects which would have occurred by
motion of the wires in one direction; the destruction of the magnetic power
corresponds to the motion of the wire in the opposite direction.

240. If, instead of intersecting the magnetic curves of a straight wire
carrying a current, by approximating or removing a second wire (235.), a
revolving plate be used, being placed for that purpose near the wire, and,
as it were, amongst the magnetic curves, then it ought to have continuous
electric currents induced within it; and if a line joining the wire with
the centre of the plate were perpendicular to both, then the induced
current ought to be, according to the law (114.), directly across the
plate, from one side to the other, and at right angles to the direction of
the inducing current.

241. A single metallic wire one twentieth of an inch in diameter had an
electric current passed through it, and a small copper disc one inch and a
half in diameter revolved near to and under, but not in actual contact with
it (fig. 39). Collectors were then applied at the opposite edges of the
disc, and wires from them connected with the galvanometer. As the disc
revolved in one direction, the needle was deflected on one side: and when
the direction of revolution was reversed, the needle was inclined on the
other side, in accordance with the results anticipated.

242. Thus the reasons which induce me to suppose a particular state in the
wire (60.) have disappeared; and though it still seems to me unlikely that
a wire at rest in the neighbourhood of another carrying a powerful electric
current is entirely indifferent to it, yet I am not awa