Electricity produced by Rotation—Direction of the Currents—Electricity from the Rotation of a Magnet—M. Arago’s Experiment explained—Rotation of a Plate of Iron between the Poles of a Magnet—Relation of Substances to Magnets of three Kinds—Thermo-Electricity.

M. Arago discovered a source of magnetism in rotatory motion. If a circular plate of copper be made to revolve immediately above or below a magnetic needle or magnet, suspended in such a manner that it may rotate in a plane parallel to that of the copper plate, the magnet tends to follow the circumvolution of the plate; or, if the magnet revolves, the plate tends to follow its motion; so powerful is the effect, that magnets and plates of many pounds weight have been carried round. This is quite independent of the motion of the air, since it is the same when a pane of glass is interposed between the magnet and the copper. When the magnet and the plate are at rest, not the smallest effect, attractive, repulsive, or of any kind, can be perceived between them. In describing this phenomenon, M. Arago states that it takes place not only with metals, but with all substances, solids, liquids, and even gases, although the intensity depends upon the kind of substance in motion. Experiments made by Dr. Faraday explain this singular action. He found that, if a piece of metal or a metallic wire forming a circuit of any form be moved from right to left across the lines of force proceeding from the pole of a bar magnet, these lines of force induce a current of electricity flowing in one direction; and when the motion of the metal or wire is reversed, the direction of the current is reversed also: the rotation of the magnet about its axis has no effect on these results, and no current is induced when the metal or wire is at rest. A plate of copper, twelve inches in diameter and one fifth of an inch thick, was placed between the poles of a powerful horseshoe magnet, consequently crossing the magnetic lines of force at right angles, and connected at certain points with a galvanometer by copper wires. When the plate was at rest no effect was produced; but as soon as the plate was made to revolve rapidly the galvanometer needle was deflected sometimes as much as 90°, and by a uniform rotation the deflection was constantly maintained at 45°. When the motion of the copper plate was reversed, the needle was deflected in the contrary direction, and thus a permanent current of electricity was evolved by an ordinary magnet. The intensity of the electricity collected by the wires, and conveyed by them to the galvanometer, varied with the position of the plate relatively to the poles of the magnet.

The motion of the electricity in the copper plate may be conceived by considering that, merely by moving a single wire, like the spoke of a wheel, before a magnetic pole, a current of electricity tends to flow through it from one end to the other. Hence, if a wheel be constructed of a great many such spokes, and revolved near the pole of a magnet in the manner of the copper disc, each radius or spoke will tend to have a current produced in it as it passes the pole. Now, as the circular plate is nothing more than an infinite number of radii or spokes in contact, the currents will flow in the direction of the radii if a channel be open for their return; and, in a continuous plate, that channel is afforded by the lateral portions on each side of the particular radius close to the magnetic pole. This hypothesis is confirmed by observation; for the currents of positive electricity set from the centre to the circumference, and the negative from the circumference to the centre, and vice versÂ, according to the position of the magnetic poles and the direction of rotation; so that a collecting wire at the centre of the copper plate conveys positive electricity to the galvanometer in one case, and negative in another; that collected by a conducting wire in contact with the circumference of the plate is always the opposite of the electricity conveyed from the centre. It is evident that, when the plate and magnet are both at rest, no effect takes place, since the electric currents which cause the deflection of the galvanometer are only induced by motion across the magnetic lines of force. When the plate is placed edgewise so as to be parallel to these lines of force, no revolution of it with the most powerful magnet produces the slightest signs of a current at the galvanometer. The same phenomena may be produced by electro-magnets. The effects are similar when the magnet rotates and the plate remains at rest. When the magnet revolves uniformly about its own axis, electricity of the same kind is collected at its poles, and the opposite electricity at its equator.

The phenomena which take place in M. Arago’s experiments may be explained on this principle. When both the copper plate and the magnet are revolving, the action of the induced electric current tends continually to diminish their relative motion, and to bring the moving bodies into a state of relative rest; so that, if one be made to revolve by an extraneous force, the other will tend to revolve about it in the same direction, and with the same velocity.

When a plate of iron, or of any substance capable of being made either a temporary or permanent magnet, revolves between the poles of a magnet, it is found that dissimilar poles on opposite sides of the plate neutralize each other’s effects, so that no electricity is evolved; while similar poles on each side of the revolving plate increase the quantity of electricity, and a single pole end-on is sufficient. But when copper, and substances not sensible to ordinary magnetic impressions, revolve, similar poles on opposite sides of the plate neutralize each other; dissimilar poles on each side exalt the action; and a single pole at the edge of the revolving plate, or end-on, does nothing. This forms a test for distinguishing the ordinary magnetic force from that produced by rotation. If unlike poles, that is, a north and south pole, produce more effect than one pole, the force will be due to electric currents; if similar poles produce more effect than one, then the power is not electric. These investigations show that there are really very few bodies magnetic in the manner of iron. Dr. Faraday therefore arranges substances in three classes, with regard to their relation to magnets:—those affected by the magnet when at rest, like iron, steel, and nickel, which possess ordinary magnetic properties; those affected when in motion, in which electric currents are evolved by the inductive force of the magnet, such as copper; and, lastly, those which are perfectly indifferent to the magnet, whether at rest or in motion.

It has already been observed that three bodies are requisite to form a galvanic circuit, one of which must be fluid. But, in 1822, Professor Seebeck, of Berlin, discovered that electric currents may be produced by the partial application of heat to a circuit formed of two solid conductors. For example, when a semicircle of bismuth, joined to a semicircle of antimony, so as to form a ring, is heated at one of the junctions by a lamp, a current of electricity flows through the circuit from the antimony to the bismuth; and such thermo-electric currents produce all the electro-magnetic effects. A compass needle, placed either within or without the circuit, and at a small distance from it, is deflected from its natural position, in a direction corresponding to the way in which the electricity is flowing. If such a ring be suspended so as to move easily in any direction, it will obey the action of a magnet brought near it, and may even be made to revolve. According to the researches of M. Seebeck, the same substance, unequally heated, exhibits electrical currents; and M. Nobili observed, that in all metals, except zinc, iron, and antimony, the electricity flows from the hot part towards that which is cold. That philosopher attributes terrestrial magnetism to a difference in the action of heat on the various substances of which the crust of the earth is composed; and, in confirmation of his views, he has produced electrical currents by the contact of two pieces of moist clay, of which one was hotter than the other.

M. Becquerel constructed a thermo-electric battery of one kind of metal, by which he has determined the relation between the heat employed and the intensity of the resulting electricity. He found that, in most metals, the intensity of the current increases with the heat to a certain limit, but that this law extends much farther in metals that are difficult to fuse, and which do not rust. The experiments of Professor Cumming show that the mutual action of a magnet and a thermo-electric current is subject to the same laws as those of magnets and galvanic currents; consequently all the phenomena of repulsion, attraction, and rotation may be exhibited by a thermo-electric current. M. Botto, of Turin, has decomposed water and some solutions by thermo-electricity; and the Cav. Antinori of Florence succeeded in obtaining a brilliant spark with the aid of an electro-dynamic coil.

The principle of thermo-electricity has been employed by MM. Nobili and Melloni for measuring extremely minute quantities of heat in their experiments on the instantaneous transmission of radiant heat. The thermo-multiplier, which they constructed for that purpose, consists of a series of alternate bars, or rather fine wires of bismuth and antimony, placed side by side, and the extremities alternately soldered together. When heat is applied to one end of this apparatus, the other remaining at its natural temperature, currents of electricity flow through each pair of bars, which are conveyed by wires to a delicate galvanometer, the needle of which points out the intensity of the electricity conveyed, and consequently that of the heat employed. This instrument is so delicate that the comparative warmth of different insects has been ascertained by means of it.

The conservation of force is strictly maintained throughout the whole science and different forms of electricity. In static electricity the positive and negative forces exactly balance one another; they are always simultaneous, and related often by curved lines of force; there is no defect or surplus, and the existence of one kind without the other is utterly impossible—it is absolutely a dual force. The very same may be said of electric currents, whether produced by the Voltaic battery or in any other way—the current in one part of the circuit is absolutely the same in amount and dual character as the other; and in the insulated Voltaic battery, where the sustaining power is internal, not the slightest development of the forces of either of these can occur till the circuit is completed or induction allowed at the extremities; for if when there is no circuit the induction be prevented, not merely no current, but no quantity of electricity at the poles ready to produce a current, can be evolved in the slightest degree.^[17]