be
brought into existence unless provision be made at the ends of the wire for
its discharge and renewal.

257. If a second wire move in the same direction as the first, the same
power is exerted upon it, and it is therefore unable to alter the condition
of the first: for there appear to be no natural differences among
substances when connected in a series, by which, when moving under the same
circumstances relative to the magnet, one tends to produce a more powerful
electric current in the whole circuit than another (201. 214.).

258. But if the second wire move with a different velocity, or in some
other direction, then variations in the force exerted take place; and if
connected at their extremities, an electric current passes through them.

259. Taking, then, a mass of metal or an endless wire, and referring to the
pole of the magnet as a centre of action, (which though perhaps not
strictly correct may be allowed for facility of expression, at present,) if
all parts move in the same direction, and with the same angular velocity,
and through magnetic curves of constant intensity, then no electric
currents are produced. This point is easily observed with masses subject to
the earth's magnetism, and may be proved with regard to small magnets; by
rotating them, and leaving the metallic arrangements stationary, no current
is produced.

260. If one part of the wire or metal cut the magnetic curves, whilst the
other is stationary, then currents are produced. All the results obtained
with the galvanometer are more or less of this nature, the galvanometer
extremity being the fixed part. Even those with the wire, galvanometer, and
earth (170.), may be considered so without any error in the result.

261. If the motion of the metal be in the same direction, but the angular
velocity of its parts relative to the pole of the magnet different, then
currents are produced. This is the case in Arago's experiment, and also in
the wire subject to the earth's induction (172.), when it was moved from
west to east.

262. If the magnet moves not directly to or from the arrangement, but
laterally, then the case is similar to the last.

263. If different parts move in opposite directions across the magnetic
curves, then the effect is a maximum for equal velocities.

264. All these in fact are variations of one simple condition, namely, that
all parts of the mass shall not move in the same direction across the
curves, and with the sam