Page:The New International Encyclopædia 1st ed. v. 12.djvu/774

MAGNETISM. 690 MAGNETOMETER. Historical Sketch. Tlie discovery of the change in the declination from point to point on the earth's surface is generally attributed to Columbus, 1492. Robert Xorinan. an instrument maker in London, first noticed the dip of a mag- netic needle in 1.570. The first real investigation of magnetic phenomena, and in particular the first theory of terrestrial magnetism, is due to Gilbert, 1000. The first niugnetic charts ever published were those of Halley — lines of equal declination — in 1701, the result of two long voy- ages. The diurnal variation was discovered in 1722 by tiraham, an instrument maker in Lon- don. The idea of taking systematic observations of the magnetic elements at difTerent points on the earth is due to Humboldt. The first to make accurate observations of all the elements was, however, Gauss. The most important modern work is that of Schmidt, Schuster, Rucker, Jlas- South Mountain, Pa., and in Virginia and North Caiolina. It is usually a.ssociated with ArehiBan gneisses and crystalline limestones. Deposits of magnetite are also known in Minnesota, Mis- souri, Colorado, Utah, California, and in foreign, countries. The important iron industry of Nor- way and Sweden is largely based upon magnetite ores. As a rock-forming mineral it is almost luiiversally distributed throughout the igneous, sedimentary, and metamorphic rocks. The pro- duction of magnetite in the United States in 1901 was 1,813,076 tons. See Iron. MAGNETO-ELECTRIC MACHINE. A ma- chine for generating electricity by rotating a coil of wire in the field of a permanent magnet usual- ly of the horse-shoe type. The principle of the instrument will be found described under Dy- namo-Electric Machinery, and it is sufficient here to mention a few of its most important uses^ Chart showing lines of equal magnetic Inclination for 1905 (British Admiralty). inclination and broken lines decreasing. Full lines indicate Increasing cart, and Bauer. Consult: Mascart, Magn6tisine terreslrc (Paris, 1900); Terrestrial Magnetism, a journal published by Dr. L. A. Bauer, Balti- more, Md. ; and the various reports and other publications of the Division of Terrestrial Mag- netism of the United States Coast and Geodetic Survey, especially The United States Magnetic Declination Tables and Isogonic Charts and Prin- cipal Facts Relating to the Earth's Magnetism (Washington, 1002). MAGNETITE, or Magnetic Iron Ore. An oxide of iron (FcjO,) containing 72.4 per cent, metallic iron. It has an iron-black color, semi- metallic lustre, hardness of from 5.5 to G.5 on the mineral scale, and a specific gravity of from 4 to 5. The crj'stalline form is isometric, com- monly octahedra and dodecahedra, but it also oc- curs massive and in granular aggregates. Mag- netite is readily distinguished from other ores by its strong magnetic properties; and in some cases it also exhibits polarity, being then known as lodestone. It is one of the most important com- mercial sources of iron, and is extensively mined in many parts of the world. In the United States it occurs in large deposits along the Ap- palachians, especially in the Adirondacks, in the Highlands of New York and New Jersey, at A magneto machine is usually employed in blast- ing to detonate the charge. By means of a rack and pinion attached to the armature the latter is rapidly rotated and a current generated which heats a platinum wire and explodes a fulminate cap. (See Blasting.) Even more familiar is the magneto call bell used in connection with the telephone (q.v. ). By turning the crank a large toothed wheel gearing into a smaller toothed wheel on the armature shaft is revolved, and a current generated which causes the bell at the distant station to ring. Magneto-electric ma- chines are also emploj'ed for generating small currents for medical use. See Dynamo-Electric Machinery. MAG'NETOM'ETER ( from Gk. liiyvns, mag- ncs, magnet + ixirpov, metron, measui-e). Any compa.ss needle so mounted that its angular de- flections may be measured is a magnetometer. It may be a simple small magnetic needle supported upon a point and swinging over a divided circle, or it may be a large magnet hanging upon a fibre of silk or quartz or other material, and the deflections read with a mirror on the magnet and using a telescope and scale or a reflected beam of light. In a simple magnetometer the magnet hangs in a small house or box of wood or copper