CHAPTER XXXI. ELECTRO-MAGNETS-DIAMAGNETISM AND MAGNE-CRYSTALLIC ACTION. If an insulated wire be wound round a bar of iron or steel, and an electric current be sent round the wire, the core becomes a magnet with its marked end in the position in which the marked end of a permanent magnet would come to rest, if it were placed, free to turn, inside the helix instead of the iron core. When the core is of soft iron it becomes magnetized on the passage of the current, and loses its magnetism when the current ceases. When the bar is of steel, it resists the assumption of the magnetic state, but, when magnetized, retains its magnetism for an indefinite time after the cessation of the current. The property of soft iron is taken advantage of in the construction of "electro-magnets," by means of which far greater magnetic forces than are given by any steel magnets are obtained, with the additional advantage that they are under perfect control, for within certain limits the strength of the temporary ironmagnet is proportional to the strength of the current in the wire. Electro-magnets are frequently made in the horse-shoe form, and when in this shape usually consist of two parallel pillars of soft iron, connected at one end by a massive cross-bar of the same metal. The wires are wound on hollow brass reels, which can be lifted on and off the iron pillars. A magnet in the possession of the author consists of a "horseshoe," fig 153, whose pillars are each 13 inches long and 24 inches diameter; the helices, which are 12 inches long and 5 inches external diameter, each contain about 1000 turns of insulated copper wire of about No. 18 gauge. The helices weigh about 35 lbs. each. Such a magnet, if fixed with its poles down ward, and excited by a powerful current, would probably carry a weight of from one to three tons attached to the armature. Magnets of this size, however, are used for a different purpose. Their magnetic action is so intense that it affects almost all known Fig. 153. substances, in addition to those few commonly known as magnetic. The action on most substances is too feeble for the attraction or repulsion to be observed directly. To observe the action, say, on a piece of glass, the magnet is placed with its pillars vertical and the cross bar at the bottom. The free poles then project up about an inch above the helices, Two blocks of soft iron, which are called the movable poles, are placed upon the tops of the pillars. A body placed between these blocks is practically between two horizontal poles. By sliding the blocks, the horizontal poles can be either approached close to each other or separated to a distance of some five inches. One end of each block is flat, so as to give the pole a vertical plane face about two inches square; the other ends are tapered to a blunt point. Either the flat or pointed ends can be turned towards each other. A body placed between the two pointed poles is subjected to more intense forces, while one placed between the flat poles is in a more uniform field of force; the action of the points being to concentrate the magnetic action in one place. In order to measure the magnetic actions of those bodies which are very feebly affected by the magnet, the torsion balance is used, being a modification of the Colomb's Balance described in Part I.* A homogeneous body being suspended between the poles by the torsion fibre will, if it is attracted, take up its position with its longest diameter pointing from pole to pole; and if it is repelled, its longest diameter pointing across the line joining the poles. Definition. The line joining the poles is called the axial line; the line at right angles to it, the equatorial. The amount of the attraction or repulsion can be measured by the number of degrees of torsion required to displace the suspended body a given number of degrees from its position of rest. Definition.-Iron and similar bodies which are attracted by the magnet are called Ferro-magnetic, or sometimes Paramagnetic bodies. Substances which are repelled are called Diamagnetic. The type of diamagnetic bodies is bismuth, which is repelled from a powerful magnet with considerable force. A small sphere inch in diameter hung by a thread, say, two feet long, between the pointed poles of a powerful magnet, may be repelled so as to move it as much as a quarter of an inch out of the vertical. The phenomena of diamagnetism were first observed by *Vol. i. p. 33. Faraday,* on a piece of the heavy glass which he had previously used for the experiments on the rotation of the plane of polarization, which will be described in Part IV. of this book. The paper containing the account of the discovery of diamagnetism was read before the Royal Society, Dec. 18, 1845, and will be found in the Phil. Trans. 1846, and in the Experimental Researches (2243). In these first experiments a rod of heavy glass was suspended between the poles of the great horse-shoe magnet of the Royal Institution. It was found that the bar always placed itself equatorially, that is, at right angles to the line joining the poles, and that it was in stable equilibrium in that position. There was also another position of rest when the length of the bar was exactly axial; but in this case the equilibrium was unstable, and on the least displacement the bar moved to the equatorial position. No difference could be detected between the ends of the bar; the direction in which either end pointed when in stable equilibrium depended solely on the direction in which it was displaced from the position of unstable equilibrium, thus showing that no permanent polarity, analogous to the polarity of a steel magnet, is acquired by the glass. Faraday continued his experiments on a great number of substances, among which phosphorus showed the effect" as powerfully as heavy glass, if not more so." He also experimented on a great number of liquids. The liquids were contained in a very thin glass tube, of the shape shown in fig. 154. Fig. 154. The action of the glass tube itself had of course to be allowed for, but, by making the tube of very thin flint glass, its action could be made very small, and, by alternate experiments with it "Isolated observations by Brugmans, Becquerel, Le Baillif, Saigy, and Leebeck had indicated the existence of a repulsive force exercised by the magnet on two or three substances, but these observations which were unknown to Faraday had been permitted to remain without extension or examination." Tyndall," Faraday as a Discoverer," p. 110. full and empty, the proper action of the liquid could be easily determined. The shape of the tube obviated the necessity of closing it-an important consideration, as cork, sealing-wax, and other substances are generally magnetic unless special care has been taken to prepare them free from iron. The following is a list of substances which Faraday found to be diamagnetic : Rock crystal. Sulphate of baryta. Muriate of ammonia. Carbonate of soda. Iceland spar. Acetate of lead. Tartrate of potash and antimony. Tartrate of potash and soda. Tartaric acid. Citric acid. Olive oil. Oil of turpentine. Jet. Caoutchouc. Sugar. Starch. Gum arabic. Wood. Ivory. Water. Alcohol. Solutions of various alkaline and earthy salts. Glass. Phosphorous. Resin. Caffeine. Margaric acid. Wax from shellac. Sealing wax. Mutton, dried. Beef, fresh. Beef, dried. Blood, fresh. Blood, dried. Leather. Apple. Bread. Iron, nickel, and cobalt were certainly paramagnetic, while platinum, palladium, and titanium showed paramagnetic effects, |