MAGNETISM, TERRESTRIAL. The fact that the earth has magnetic properties over its sur face is shown by the behavior of a freely sus pended magnetic needle, which at any one place on the earth's surface takes a definite direction, and so indicates the direction of the magnetic line of force at that point due to the earth's action. Owing to the great size of the earth, `magnetic storms,' when sudden and most vio lent changes in the elements take place. The occurrence of these storms obeys the same law as that of sun-spots on the sun; and the ex planation of neither is as yet known. In addi tion to the periodic changes for each day and each year, there is a 'secular' change of all the elements which has not yet been shown to. be periodic. If one were to look down a freely sus pended magnetic needle from its centre. it would seem to point to some point on the surface of the earth. The position of this point is slowly mov ing from year to year and since the year 1540 it has moved in approximately an ellipse over about two-thirds of the circumference in the direction of the hands of a watch. if nothing happens to disturb the orderly march of events, the magnetic elements at London in about the year 2010 will be the same as they were in 1540.
The simplest and most instructive manner of considering the magnetic properties of the earth is to imagine drawn on its surface: its magnetic field in any limited space—e.g. a few cubic meters—is uniform. The intensity of this field at any point and its direction are called the 'magnetic elements' of that point. To locate the direction the simplest method is to consider a plane drawn through the centre of the earth and the freely suspended magnetic needle—this is the 'magnetic meridian'—and another plane drawn horizontally through the middle of the needle. The angle made by the magnetic merid ian and the geographic meridian is called the 'declination': that made by the direction of the needle and the horizontal plane, the 'inclination' or 'dip.' All three of these quantities. the in tensity, the declination, and the inclination, can be determined at any one point on the earth's surface. As they are all varying slightly continu ously, in most magnetic observatories automatic self-registering devices are used to secure records of these fluctuations. See MAGNETOMETER.
There are daily vibrations of the elements, annual ones, and others also. That is, the daily variation is a gradual change in one direction and then a slow return. There are so-called (1) A series of lines such that at any point of one line the declination is the same. These
are called 'isogonals' or isogonie lines (q.v.).
(2) A series of lines such that at any point of one line the inclination is the same. These are called 'isoclinals' (q.v.). The 'magnetic poles' are defined to be those points where the dip is 90'.
(3) A series of lines such that at any point of one line the intensity of the field is the same. These are called 'isodynamie lines' (q.v.).
There are other lines which may be drawn. but these three sets are the most important. They are changing from year to year owing to the 'secular variation.' The origin of the magnetic action of the earth is not known: but it un doubtedly has some connection with the fact that the earth is a huge portion of matter rotating rapidly on its axis. It can be shown that only a portion—abont one-fortieth—of the magnetiza tion of the earth can be due to causes external to itself ; undoubtedly. however. some of the fluctua tions of the magnetic elements are due to currents of electrically charged particles in the atmos phere. See ATMOSPHERIC ELECTRICITY.
Instonicm. SEETC11. The discovery of the change in the declination from point to point on the earth's surface is generally attributed to Columbus, 1492. Robert Norman, an instrument maker in London, first noticed the dip of a mag netic needle in 1576. The first real investigation of magnetic phenomena, and in particular the first theory of terrestrial magnetism, is due to Gilbert, 1600. The first magnetic 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 Graham, an instrument maker in Lon don. The idea of taking systematic observations of the magnetic elements at different 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, Rficker, Mas South Mountain, Pa., and in Virginia and North Carolina. It is usually associated with Archaean gneisses and crystalline limestones. Deposits of magnetite arc 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 universally 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.