Page:The New International Encyclopædia 1st ed. v. 12.djvu/766

* MAGNESIUM LIMESTONE. 684 MAGNETISM. MAGNESIUM LIMESTONE. See DoLO- jiriK. MAGNET. A substance which has the power of altracliii'.' iron, nickel, cobalt, and certain other substances. A niafrnet may be either natu- ral, as in the case of lodestone. or artificial, as in the case of magnets produced from iron or steel, ijee Mac.nktis.m. MAGNETIC CURE (from Lat. magncs, from Gk. iiiyirris. magnet, from Mayvrjala, Magiusia, a district in Thessaly, where magnetic stones abounded). It was held by physicians of old that the magnet exercised an important influence on the human body, or on the bodies of certain persons: this being shown in the alleviation of headache, toothache, cramps, etc. It has. how- ever, been proved that the magnet as such has no influence on animal organisms, and that ac- cordingly all cures ])rofessedly resting on such action have been due to delusion or deceit. But it is quite otherwise with magneto-electricity and galvanism. See Electricity. Medical, Uses of. Consult Peterson and Kenelly, "Magnetism," in Xctc York Medical Journal (August S, 1892). MAGNETIC ELEMENTS. See Magnetism, Teuukstiual. MAGNETIC EQUATOR. An imaginary line surriiuiuliiig the eartli near the geographical equator on which the longitudinal axis of a freely suspended homogeneous magnet, sym- nu'lrically magnetized, lies in a horizontal plane without flip (q.v.). It is a rather irregular curved line crossing the equator in several places, but never receding from it more than about 12 degrees. It is shown on the map of equal mag- netic inclination in the article Magxetlsm, Ter- KE.STRIAL. See CoMi-ASS; also ilAGXETlsii, Ter- bestrial; and Deviation. MAGNETIC MERIDIAN. See ^Ugxetism, Tehues iTUAi. ; Compass. MAGNETIC MOUNTAIN. In the Anihinn Siilhlfi, a mountain which attracted passing ships and by its magnetic jiower drew out the nails and fastenings, and so destroyed the vessels. The tale appears in various works of literature. MAGNETIC NEEDLE. See Compass ; Devi- ation: Mai;netism. Tekre.strial; Magnetism. MAGNETIC NORTH. See JUg.n-etism, Tekhestrial: Compass. MAGNETIC POLE, UNIT, Definitiox of. See Magnetism: and Klecthicity, section Laics of Slridi/ EUclric Currents. MAGNETISM. The name 'magnet' is given any body which possesses the power of attracting pieces of iron. There are certain natural ores which have this power, but all magnets actually in use are artificial. The origin of the use of a magnetic compass as a means of obtaining approximately the direction of the geographical North Pole of the earth is unknown, but the first scientific work on the laws of magnets was done by William Gilbert (1540-1003). who published his researches and ideas in his great book De Mnfinrlr. Since his time the most important work has been done in connection with electric currents. (See Electricity.) Several attempts have been made to explain magnetism, that is, to explain the magnetic action of a molecule of a magnetic substance by some electrical theory. Amp&re advanced the idea that in each such molecule there is an electric current flowing in a tixed path. This idea is not. however, in accord with modern conceptions of the nature of a molecule. Weber tried to account for diamag- netism by the idea of induced electric currents in the molecules. A more modern theory is one proposed tentatively by J. J. Thomson, in which molecular magnetism is regarded as due to rota- tion of the molecule with its Fardday tubes connecting the atoms. In addition to attracting iron, magnets attract pieces of other kinds of matter, e.g. nickel, cobalt, manganese, chromium, and a few others, while they repel small pieces of bismuth, anti- mony, silver, and a few other substances. It was shown by Faraday that the question of repulsion or attraction depended entirely on the relative magnetic properties of the substance which is being acted on by the magnet and the material medium in which it is immersed — this last is, of course, generally air. Those bodies which when surrounded by air are attracted toward a magnet are called 'magnetic' or 'para- magnetic' sulistances; while those which under similar circumstances are repelled are called 'diamagnctic' (See Diamagnetlsm.) Faraday showed, too, that there is in neither case attrac- tion or repulsion if the magnetic field is 'uni- form;' that is. if there is the same magnetic force felt at each point of the surrounding air. There are two methods in general use for making artificial magnets: one is to take a bar or a needle of a magnetic substance, e.g. iron or steel, wrap an insulated wire around it like thread on a spool, and pass a strong electric current through the wire; the other is to take as before a bar or a needle of a magnetic sub- stance and place it near a magnet. (A modifica- tion of the latter method will he described later.) The intensity of the magnetization is increased in both cases by hammering the bar while ths magnetizing action is going on. It is found that a piece of steel magnetized in this way remains so for a long time; while a piece of iron loses its magnetization. If a nuinljer of such bar magnets or magnetic needles are made, the following facts may be observed: If a magnet in the form of a bar or needle is suspended so as to be free to rotate around a vertical pivot, as in the mariners' compass (q.v.), it will turn and place itself in a generally north and south direction. The eiiil of the magnet which points toward the north is called the 'north pole' of the magnet : and the other, the 'south pole.' This proves that the earth itself has magnetic actions: and the plane which includes the centre of the earth and the direction of the magnetic needle at any point on the earth's surface is called the 'magnetic meri- dian' at that point. It should not be thought that the 'poles' of a bar magnet are definite points. Magnetic forces may be felt over the whole magnet, but of course more intensely near the ends. In any definite case the resultant of all these forces on any one outside body will pass through a certain point in the magnet; but this point will change with the position of the outside body. If this magnet is floated on water, it is found that there is no translation, simply a rotation around a vertical axis. The magnetic field due to the earth must be uniform for any limited region on the earth; and this experiment proves