ROTATION, MAGNETISM OF. This was discovered by Arago in the years 1824-25. He observed that when a magnetic needle was made to oscillate immediately above a copper plate, it came sooner to rest than it did otherwise. The oscillations were made in the same time as when away from the plate, but they were less in extent; the plate seemed thus to act as a damper to the motions of the needle. This being the action of the plate at rest on the needle iu motion, Arago reasoned that the needle at rest would be influenced by the plate in motion. Experiment continued his opinion. He made a copper disk revolve with great rapidity under a needle, resting on a bladder placed immediately above it, and quite unconnected with it, the middle of the needle being placed above the center of the disk. As expected, the needle deflected in the direction of the motion of the disk. The deflection of the needle increased with the rapidity of the motion, and when it reached a sufficient amount, the needle no longer remained in a fixed position, but turned round after the disk. This action of the revolving disk was attributed to what was then called the "magnetism of rotation," and the name has since been retained.
The explanation of this phenomenon was first made by Faraday (1832). He found it to arise front the reaction of currents, induced in the plate in motion by the magnet. The accompanying figure illustrates the electrical condition of the plate, PP is the plate, rotating in the direction indicated by the arrow; NS is the needle; and the lines with the arrow-heads indicate the general direction of the currents induced by rotation under the magnet in the plate. There are two complete circuits on each side of the disk, coinciding in the middle, and taking the direction CC. It is the con joined current which affects the needle; it runs in a direction a little in advance of the needle, as the induc tive power of the magnet takes some time to act. As the induced current lies below the needle, the deflection (according 'to Ampere's rule, see GALVANISM) takes place in the direction of the motion of the disk. When cuts are made in the disk in the line of the radii, it loses almost entirely its disthrbing power; the currents formed in the whole disk can no longer take place, and those formed in the various sectors are weak in comparison; by filling up the vacant spaces with solder, the power is nearly restored to it. As is to be expected, the effect of the revolving plate depends hn the conducting power of the material of which it is made. It is owing to its high conducting power that copper is so much' used in these
experiments; hence, also, it is that copper should be so much used in the construction of magnetic apparatus. A copper compass-box, for instance, is not only desirable, from its being free from iron, but it acts as a damper to bring the needle quickly to rest when disturbed.
The magnetism of rotation is only one of a large class of phenomena in which the motion, either of a magnet or of a conductor near it, induces an electric current in the conductor. We may here quote two experiments which may be looked upon as the converse of the magnetism of rotation. In the first experiment, a small cube of copper is hung by a thread to a frame, and placed between the poles of a powerful electro magnet; the cube is sent into rapid rotation by the twist on the thread, previously given it; it is instantly brought to a halt, when the current is allowed to circulate in the coils of the magnet, and it begins its motion 'again when the current is turned off. In the second experiment, a disk of copper is made to rotate rapidly between the poles of an electromagnet, by means of a handle and intervening wheel-works turned by the experi menter. When the current invests the soft iron poles with magnetism, the disk, moving freely before, appears suddenly to meet with an unseen resistance, and the rotation con tinues slowly or not at all. If persisted in, the rotation causes the disk to rise in tempera ture, the rise being proportionate, aeccording to Foucault, to the square of the velocity of rotation. These and all similar phenomena illustrate a law that holds universally in magnetic induction, and was first enunciated by Lenz: 'Rhea a CUreePt 18 induced by the motion of a, magnet or conductor, time inductive action, tends to develop in the conductor a rent in such a direction that its action will be to oppose the motion producing it. Thus, in the last experiment, the part 'of the disk approaching the poles has a current developed in it which repels them, and the part leaving the poles, has a current induced in it which attracts them. The approach of the one part and the departure of the other are equally opposed by the currents induced in them. The same mode of explanation applies to the other experiments referred to.