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 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 same angular velocity.  But they are forms of expression which, being retained in the mind, I have found useful when comparing the consistency of particular phenomena with general results.