DIAMAGNETISM. The fact that iron is attracted by the magnet, has been known from very remote times; that bismuth exhibits a repulsive action towards the magnetic needle, has been now known for nearly 100 years. Dr. Faraday was the first (1845) to show that all bodies are more or less affected by magnetic influence, and his beautiful researches on the subject have opened up a new field in the domain of science. He found that the magnetism of bodies was manifested in two ways—either in being attracted by the magnet, as iron; or in being repelled, like bismuth. When a needle or slender rod of iron is suspended between the poles of a magnet being attracted by them, it takes up a position of rest on the line joining the two poles. When a substance behaves itself in this manner, it is said by Faraday to be paramagnetic, and to place itself axially. A rod of bismuth, on the other hand, being repelled by the poles of the magnet, comes to rest at right angles. Bismuth, and the like substances, he calls diamagnetic, and they are said to place themselves eguatorially. These terms, being both definite and graphic, have been universally adopted. Magnetic is the term used by Faraday to indicate magnetism of either sort, although ingeneral language it is understood to refer to paramagnetic bodies, such as iron, etc. Paramagnetic bodies, then, are those which manifest the same properties with regard to the magnet that iron does; and diamagnetic bodies are those which, like bismuth, show opposite but corre sponding properties; so that in circumstances where paramagnetic bodies place them selves axially, diamagnetic bodies place themselves equatorially; and where the former are attracted, the latter are repelled, and rice versa'. A paramagnetic, therefore, not in the elongated form, but in a compact shape, such as a ball or cube, is attracted by either pole of the magnet, when suspended near it; a ball or cube of a diamagnetic, on the other hand, experiences, when so placed, repulsion. The paramagnetism of iron, nickel, and cobalt, becomes manifest in the presence of magnets of ordinary power; but the magnetism of most other substances is so feeble as to be developed only under the influence of the strongest magnets. As electromagnets far exceed permanent steel
magnets in strength, they are selected for investigations on the magnetism of bodies. The soft iron horseshoe of the clectro-magnet is enveloped towards its extremities in coils of insulated copper wire which communicate with a galvanic battery by wires. The electro-magnet is fixed in an upright wooden frame. The ends or poles of the magnet rise slightly above the table or board which forms the upper part of the frame. In order conveniently to suspend substances between the poles, and to protect them while under observation from currents of air, a glass frame of simple construction is made to fit the table. The upper plate of the frame admits a wooden ring, into which an upright glass tube is fitted. The thread by which the needle is suspended is wound round a slender movable bobbin at the top, so that it can be elevated or lowered to the proper position. To modify and direct the action of the magnet, two pieces of soft iron are made to rest on the end faces; these are pointed at one extremity, and flat at the other, so that the force of the magnet may he concentrated in the points, when they are turned towards each other; or diffused over the opposite flat surface, when their position is reversed.
To observe the effect of the magnet on liquids, Faraday placed them in long tubes of very thin glass, and suspended them as in the case of solid needles. It was found that some arranged themselves axially, and others equatorially. The attraction and repulsion that liquids experience in the presence of the magnet has been prettily shown by Plucker. A large drop of liquid is placed in a watch-glass, and laid upon two poles of the magnet. If the liquid be paramagnetic, the surface becomes depressed at the interval between the poles, and heaped up over thcPextreme edges of them. A diamag netic liquid, on the other hand, shows a depression at each edge of the poles, and a heaping up at the center.