Page:Collier's New Encyclopedia v. 06.djvu/106

MAGNESIUM 80 MAGNETISM MAGNESIUM (-ne'shium) (Latinized from Magnesia, a city in Asia Minor), in , chemistry, a diatomic metallic element; symbol, Mg; at. wt. 24.4; sp. gr. 1.743. Fuses and volatilizes at a red heat. Mag- nesic sulphate (Epsom salts) was known in the middle of the l7th century, but the metal was first isolated by Davy. The compounds of magnesium are widely dis- tributed in nature, occurring as mag- nesite, MgO"CO; ophite or serpentine, MgO"=Si20; talc, MgO'^SisOa; dolomite, kainite, epsomite, etc. MAGNETIC EQUATOR, a theoretical line drawn on the surface of the earth, connecting the points at which a dip- ping »«edle assumes a horizontal posi- tion, and at which the magnetic dip is zero. MAGNETIC NEEDLE. See MAGNET- ISM. MAGNETIC POLES. See MAGNETISM. MAGNETISM, the science which treats of the phenomena exhibited by magnets — phenomena due to one of those forces which, like electricity and heat, are known only by their effects. The phenomena of magnetism were first ob- served in the loadstone or magnet (so named from Magnesia in Asia Minor). The loadstone is a kind of iron ore (mag- netic iron ore), and is found in many parts of the world, especially in the Scandinavian peninsula, and in Siberia. It has the power of attracting small pieces of iron and steel, and when sus- pended in such a way as to be able to move freely, always points to what are called the magnetic poles of the earth, that is nearly N. and S. A piece of load- stone forms a natural magnet, and has the further remarkable power of giving all its own properties to hard iron or steel when these bodies are rubbed by it. A bar or mass of iron or steel to which the peculiar properties of a natural mag- net have been imparted by friction from other magnets or by electric induction is called an artificial magnet. When freely suspended, all magnets, natural and artificial, rest with their lengths in a N. and S. direction, and this property is utilized in the well-known compass. They attract iron and other magnetic substances with a force increasing from the middle of the magnet to its extremi- ties, which are called its poles. The mag- netism at the two poles is different, that pole which points to the N. is distin- guished as the north or north-seeking or austral pole, or by the sign plus ( + ) ; that which points to the S. as the south or south-seeking or boreal pole, or by the sign minus ( — ). The poles of the same denomination repel each other, while those of different names have mutual at- traction, thus resembling the two elec- tricities, positive and negative. The in- tensity of this attraction and repulsion varies inversely as the square of the dis- tance, a law which also governs electri- fied bodies. Magnetism pervades the earth as elec- tricity does the atmosphere. It assumes a totally diffei'ent form in different sub- stances; the metals iron, nickel, and co- balt being strongly attracted by the magnet; others such as bismuth, copper, silver, gold, etc., being as strongly re- pelled (see DiAMAGNETic) . The space in the neighborhood of a magnet is called the magnetic field; a piece of soft iron brought into this space becomes mag- netic, although it loses its magnetism as rapidly on removal from the field. (See Induction.) Steel has great coercive force, in virtue of which it requires time for magnetization, and retains its mag- netism on removal from the field. Hard steel may be made magnetic by rubbing it several times in the same direction with a powerful magnet, and hence it is easy to multiply magnets. The most powerful permanent magnets are pro- duced by rubbing bars of steel on electro- magnets (see Electro-magnetism), or by moving them backward along the axis of a coil of wire in which an electric cur- rent is passing. A bar is magnetized to saturation when its magnetism is as great as it can retain without future sensible loss. Terrestrial magnetism, which pervades the whole earth, is extremely compli- cated. It becomes manifest by its influ- ence on the magnetic needle, varying with the time and place over the earth. One pole of the needle points toward the N., the other toward the S. There are, however, only two lines on the surface of the earth on which it points directly N. and S., and where the magnetic and geographical meridians appear to coin- cide. Elsewhere the needle deviates more or less from the true N. This is termed the declination of the needle, and varies from place to place, and in the course of time at the same place (see Isodynamic). When a needle is balanced on a horizontal axis, so that it can turn in a vertical plane, the extremity at- tracted by the nearer magnetic pole of the earth points more or less downward (see Dipping Needle). The angle thus made is called the dip or inclination, and the lines marking equal inclinations on a map are called isoclinal lines. They in- tersect the isogonal lines, and the dip in- creases toward the perpendicular as the magnetic poles are neared. These mag-