CHAP. V.

ELECTRO-DYNAMIC INDUCTION.

Magnetism by Rotation.

We owe to M. Arago the first experiment, which showed that motion is a means of developing magnetism or electric currents in all bodies. We must not confound this action with what had been discovered by Coulomb, namely, that magnets are capable of acting upon all bodies generally in the same manner, except in regard to intensity, as they act upon iron. This species of action will be the subject of the following Chapter. The phenomena, with which we are engaged in the present, are those to which motion gives rise, that is to say, the change of place of the cause that acts, in relation to the body, upon which it acts. The term magnetism by rotation is applied to the form under which M. Arago first made manifest this class of phenomena.

Having oscillated a magnetised needle, freely suspended in a circular copper cage, the bottom and sides of which were very near the needle, M. Arago remarked that the oscillations of the needle rapidly diminished in extent, and very quickly ceased, as if the medium in which they were being produced had become more resistant. Furthermore, the neighbourhood of the copper exercised no other influence than upon the amplitude, and not upon the duration of the oscillations, which are accomplished in exactly the same time as in the free air. By making the needle oscillate above planes of different natures, and at variable distances from the same plane, M. Arago became satisfied that distance considerably diminished the intensity of the effect, and that metals acted with more energy than wood, glass, &c. Mr. Seebeck, who repeated M. Arago's experiment immediately after its discovery, obtained analogous results. Thus he found that if the needle

oscillated above a plane of marble, it was necessary that it should accomplish 112 oscillations, in order that the amplitude should be reduced from 45° to 10°; whilst it only required 71 oscillations above a plate of zinc, and 62 above one of copper. Unfortunately, M. Seebeck's experiments, not having been made with plates of the same dimensions, and in particular of the same thickness, the results are not comparable.

After the first experiment that we have been relating, M. Arago conceived the idea of trying whether the plate, which possessed the property of diminishing the amplitude of the oscillations of a magnetised needle without altering their duration, would not draw the needle with it, when itself put in motion. This in fact is verified.

We fix to a rotation apparatus, such as a table made for experiments on the centrifugal force, a copper disc about 12 inches in diameter, and about in. thick (Fig. 140.); above it,

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Fig. 140.

and very near, we suspend, by a silk thread, without torsion, a magnetised needle, in such a manner that its point of suspension is exactly above the centre of the disc; care is taken to interpose between the disc and the needle a screen of glass or pasteboard, so that the agitation of the air resulting from

the motion impressed upon the disc may be without influence upon the needle. The disc is put in rotation, and the needle is seen to deviate from its normal direction, in the direction of the movement impressed, and to make with the magnetic meridian a greater or less angle of deviation, according to the velocity of the rotation. If the movement becomes very rapid, the needle is finally drawn with it, and rotates with the disc itself.

The force diminishes very rapidly with the distance of the needle from the disc, in a relation, as it appears, greater than the square; the angles of deviation within certain limits are proportional to the velocity. The power of the copper disc is considerably diminished by cutting slits in it in the direction of rays: these solutions of continuity, which in no degree alter the mass, exercise a great influence upon the intensity of the action.

Independently of the preceding observations, M. Arago, by analysing the force called into play in these experiments, discovered that, from drawing or tangential to the disc which it is, it may become perpendicular to its plane, and even act in the same direction as its radii; which proves that the total force may be regarded as a resultant of three components, one perpendicular to the radius of the disc, the second perpendicular to the plane of the disc, and the third parallel to its radii. We have already proved the existence of the first in the fundamental experiment. In order to prove the existence of the second, we have merely to place a magnet vertically above, and very near to the disc, which is easily accomplished by suspending it to the scale-pan of the balance. Immediately that the disc is in motion, the scale-pan rises, which proves that the vertical magnet is repelled by the disc, and the weight necessary to be placed in the scale-pan, in order to re-establish equilibrium, indicates the energy of this repulsion. With regard to the third component, its existence is demonstrated by placing vertically over the disc a needle, movable, like the dipping needle, around a horizontal axis, and in such a manner that this axis is perpendicular to the radius of the disc. If the needle is over the centre, it experiences no

action, neither does it experience any if it is situated over a point nearer to the edge than to the centre of the disc, namely, at a distance from the centre equal to about two-thirds of the radius. But between this point and the centre, the lower pole of the needle is constantly attracted towards the centre; beyond this point, namely, between this point and the edge, the pole is attracted towards the edge.

Whilst M. Arago was analysing, as we have just seen, the force that he had discovered, MM. Babbage and Herschel, Harris, Barlow, and others, endeavoured to study the causes which may make the intensity vary and may modify it. MM. Babbage and Herschel had repeated M. Arago's experiment by inverting it. They had found that discs of copper or other substances, when freely suspended over a horse-shoe magnet put into rotation, turned in the same direction as the magnet, with a movement at first slow, but whose rapidity gradually increased. The magnet was so arranged as to be able to receive a rapid movement around its symmetrical axis, placed vertically with the poles upward. The movement of the disc changed in direction with that of the magnet. The interposition of plates of glass and of nonmagnetic metallic plates in no degree modified the effects: it was not the same when the interposed plates were of iron; the action was then extremely reduced, or even entirely annihilated, when there were two plates one above the other.

By subjecting to experiment discs of the same diameter but of various natures, the two philosophers found great differences between these discs with regard to their faculty of being drawn onward by the magnet in rotation, although the velocity of the movement impressed and their distance from the magnet were the same. Thus, zinc and copper appear in this respect to possess an energy four times more powerful than lead, and a hundred times more powerful than antimony. But the discs unfortunately were not all of the same thickness, whicn renders the results of the experiments but little comparable as, within certain limits, thickness exercises a marked influence over the intensity of the phenomena. It is probably to this cause that we must attribute the little power

that MM. Babbage and Herschel had found in gold. On the other hand, they did not obtain any effect with wood, glass, resin, sulphur, and sulphuric acid.

These two philosophers verified the accuracy of M. Arago's observations on the influence of solutions of continuity, either parallel or total, in the masses subjected to experiment. Thus, a light disc of copper, suspended at a given distance above a magnet, executed its revolutions in 55". When cut in eight places, in the direction of a radius near the centre, it required, under these circumstances, 121" to execute the same number of revolutions. But the cut parts having been soldered in again with tin, the primitive effect was almost entirely re-established; in such sort, that the disc was able to make its six revolutions in 57"; that is, almost in the same time as when it had not been cut. The same effects were in a similar manner obtained with the other metals, employed either as discs or solderings. We shall soon see that all these results, as well as the effects themselves, of magnetism by rotation, are very readily explained by the presence of electric currents, that are determined in the disc by its vicinity to a magnet, and the circulation of which is prevented when there are solutions of continuity in the conducting body. Thus no action is obtained when we rotate in relation to each other two discs of any kind, or a piece of soft iron or non-magnetised steel, and a disc of metal. It is absolutely necessary that one of the bodies in motion shall be a magnet.

Sir W. Harris, who made a great number of experiments on the same subject, not only found great differences between bodies with regard to the faculty they possess of drawing onward the needle, but also with regard to the property they possess of intercepting this action. He recognised that iron, and magnetic substances generally are not the only ones that are thus able to arrest the effect of magnetism by rotation. Only it is necessary to give a very great thickness (from 3 to 5 in.) to the plates of non-magnetic substances, such as copper, silver, and zinc, in order that, when interposed, they may arrest the action of a magnet in motion upon a movable