ential instead of
direct.

125. But it is possible (though not necessary for the rotation) that _time_
may be required for the development of the maximum current in the plate, in
which case the resultant of all the forces would be in advance of the
magnet when the plate is rotated, or in the rear of the magnet when the
latter is rotated, and many of the effects with pure electro-magnetic poles
tend to prove this is the case. Then, the tangential force may be resolved
into two others, one parallel to the plane of rotation, and the other
perpendicular to it; the former would be the force exerted in making the
plate revolve with the magnet, or the magnet with the plate; the latter
would be a repulsive force, and is probably that, the effects of which M.
Arago has also discovered (82.).

126. The extraordinary circumstance accompanying this action, which has
seemed so inexplicable, namely, the cessation of all phenomena when the
magnet and metal are brought to rest, now receives a full explanation
(82.); for then the electrical currents which cause the motion cease
altogether.

127. All the effects of solution of metallic continuity, and the consequent
diminution of power described by Messrs. Babbage and Herschel[A], now
receive their natural explanation, as well also as the resumption of power
when the cuts were filled up by metallic substances, which, though
conductors of electricity, were themselves very deficient in the power of
influencing magnets. And new modes of cutting the plate may be devised,
which shall almost entirely destroy its power. Thus, if a copper plate
(81.) be cut through at about a fifth or sixth of its diameter from the
edge, so as to separate a ring from it, and this ring be again fastened on,
but with a thickness of paper intervening (fig. 29.), and if Arago's
experiment be made with this compound plate so adjusted that the section
shall continually travel opposite the pole, it is evident that the magnetic
currents will be greatly interfered with, and the plate probably lose much
of its effect[B].

[A] Philosophical Transactions, 1825, p. 481.

[B] This experiment has actually been made by Mr. Christie, with the
results here described, and is recorded in the Philosophical
Transactions for 1827, p. 82.

An elementary result of this kind was obtained by using two pieces of thick
copper, shaped as in fig. 28. When the two neighbouring edges were
amalgamated and put together, and the arr