Many years ago the motions of a magnet under the influence of a rotating copper-plate, known as " Arago's Rotations," excited consider able speculation and inquiry [MAGNETISM], until Faraday gave the true explanation. The leading experiment may be shown by suspending a bar magnet freely by its centre, so that it can move in a horizontal plane, while beneath it a circular disc of copper being made to rotate, the magnet will move round in the same direction as the disc, even though a sheet of paper or of glass be interposed between them. Discs of other metals on being rotated give motion to the magnet, but none so readily as copper. A narrow strip cut out of the disc, from the circumference to the centre, is sufficient to prevent the disc having any action on the magnet when the former is made to rotate. If the cut edges of the disc be connected by a piece of wire, the power of the disc over the magnet is completely restored.
In investigating these phenomena, Faraday found that when a con ducting body is passed before the pole of a magnet, or between its opposite poles, so as to cut the magnetic curves at right angles, electrical currents are produced across the metal, transverse to the direction of motion. If, for example, the copper disc c, be made to revolve in the direction of the arrows at the edge, between the poles N B of a horse shoe magnet, and a wire to, connected with one end of the galvano meter g, be pressed against the centre of the disc, while the other wire w', proceeding from the galvanometer, rests against the edge of the disc, between the magnetic poles, a current will flow from the centre to the circumference of the disc, and then through the wires in the direction of the arrows. If the disc revolve in the opposite direc tion the current will (row from the edge to the centre of the disc. Now in Arago'a experiment, when the copper disc revolves beneath the magnet, it cuts the magnetic curves at right angles (adopting Ampere's view that a series of electric currents are perpetually circulating around the component particles of a bar magnet, in planes at right angles to the magnetic axis), in which case currents are produced beneath the north pole from the centre of the plate towards the circum ference. These currents proceed from the circumference to the centre, beneath the south pole, traversing the diameter of the plate, parallel to the magnet, and return by the more distant parts of the plate. Such currents exert a repulsive action on the magnet in a direction coinciding with the motion, and there are no currents until the magnet or the plate be in motion.
More convenient arrangements than the above have been contrived for the generation of magneto-electric currents, the best known of which is Saxton's machine. It consists of a powerful
horse-shoe magnet placed horizontally upon one of its sides, and oppo site and nearly in contact with its poles an armature of soft iron a b is made to revolve upon a horizontal axis A. In the figure the horse-shoe magnet is not shown. The armature consists of two pieces of iron, about 2 inches long, attached to a cross-piece of iron x, at such a dis tance as to be opposite the middle of each pole of the horse shoe magnet. Each piece of iron, a b, contains a coil, c d, of fine copper wire covered with silk to insulate the coils. The corresponding ends of each of these coils are connected together ; one pair, e f, is attached to the spindle s, on which the armature revolves, and through it is con nected with a circular copper disc, i, the edge of which dips into a cup of mercury, m, while the other pair of wires, g h, is connected with a stout piece of copper that passes through the axis of the spindle s, from which it is electrically insulated, and terminates in a slip of oopper, k, which is plaoed nearly at right angles to the cross-piece x. Beneath k; is a second cup of mercury, 1, which can be made to com municate with the cup m by a wire or some other conductor. During the revolution of the armature, the points of the slip le alternately dip into the mercury and rise above it. Now when 1 and m aro con netted, and the point k beneath the mercury, the circuit is complete. The ends of the armature, a b, are exactly opposite the ends of the horse-shoe magnet, but by giving motion to the axis they quit that position, lose their magnetism, the current is broken, the slip k leaves the mercury, and in doing so produces a bright spark. If the con nection between the mercury cups be made by means of wires, termi nating In copper cylinders, held one in each hand, a rapid succession of powerful shocks will be felt on causing the armature to rotate. Water and saline solutions may be decomposed by transmitting these currents through them. For currents of high intensity, such as are required for these decompositions, the armature should be furnished with a great length of thin wire ; but for the display of large sparks, and the ignition of platinum wire, &c., a shorter length of thicker wire may be used. It should be observed that the currents produced during each half revolution in the two limbs of the armature are in opposite direc tions, but in some machines arrangements are made by means of com mutators rrELEORAPH, ELECTRIC] for keeping up a continuous current in direction. This is of importance, since magneto-electric machines now often take the place of the voltaic battery in the electric telegraph, and in the recess of eleetro-silvering, and electro-gilding