TERRESTRIAL MAGNETISM, the science pertaining to the study of the magnetic phenomena of the earth.
It is probable that the directive property of loadstone was dis covered independently in Europe and in China, but at a much earlier date in the latter than in the former. It is unlikely that the characteristic was known in Europe before the tenth century, but the Chinese were aware of it before the Christian era. Some go so far as to say that it was known in China as early as 2634 B.C., and there is evidence of the use, as early as III° B.C., of "south pointing carts," i.e., carts carrying a pivoted dummy with out stretched arm, and containing a magnet which caused the arm to point south. It has been claimed that, as early as the end of the eleventh century, the Chinese were aware of the fact that the compass needle does not point to the true north, but, as far as western civilization is concerned, Columbus is usually, although not universally') (Note: the superior figures in the text refer to the latter part of the bibliography at the end of this article) cred ited with the discovery during his voyage to America in 1492.
The discovery of the "magnetic dip" is usually attributed to Robert Norman, an English instrument maker who describes it in The Newe Attractive, published in 1581, and who constructed a form of dip circle and found a rough value for the dip in London. The secular change in the orientation of the compass needle was discovered and recorded in 1635 by Henry Gellibrand as a result of the measurements which had been made at Limehouse by William Borough in 1581, by Edmund Gunter in 1622, and by himself in 1633. (See COMPASS.) The fact that the orientation of the compass varied through out the day was discovered by George Graham in 1722, and a more careful analysis of the phenomenon was made from 1756— 1759 by j. Canton. About this time also a connection between magnetic disturbances and aurora was noticed by Canton and Peter Vilhelm Wargentin.
Not until the end of the eighteenth century was it known that the earth's total magnetic intensity varied over its surface. The
first to observe the phenomenon was a Frenchman, Paul de Lamanon, but the first published results are those of the traveller Humboldt who made observations between 1798 and 1803, using measurements of the time of oscillation of a dipping needle as a means of comparison of the total intensities at different places. The diurnal variation in the dip and in the horizontal intensity of the earth's magnetism was discovered by Arago in 1827.
The foundations of absolute methods for measuring terrestrial magnetic quantities were laid by Gauss, who initiated regular observatory work iii terrestrial magnetism at Gottingen in Magnetic observatories were established at St. Helena, Cape of Good Hope, Hobart, and Toronto by the middle of the 19th cen tury, and since that time numerous observatories have sprung up in all parts of the world.
General Considerations.—The magnetic field of the earth is roughly that of a uniformly magnetized sphere, and the field of such a sphere at external points is the equivalent of that due to a magnetic doublet placed at its centre. According to an analysis of L. A. the values M of the resultant moment, Mp of the axial component and of the equatorial component for this doublet are as follows: M = 8.04X 1o" c.g.s. ; Mp = 7.88X 1o" c.g.s. ; 1.60 X 1o" c.g.s. The axis of the doublet intercepts the northern hemisphere in latitude 78° 32' N. and longitude 69° o8' W. The average intensity of magnetization, or moment per cubic centimetre of the earth is 0.074 c.g.s. unit.
The magnetic poles of the earth are defined as the places where the magnetic lines of force are vertical. Only very roughly may they be regarded as situated at the intersection of the prolonga tion of the axis of the doublet with the earth's surface. From observations taken between 1903 and 1906, R. Amundsen placed the north magnetic pole at 7I° N. latitude and 96° W. longitude. The probable position of the south magnetic pole as calculated from data obtained on the "Discovery," 1902-4 is 72° 50' S. lati tude, and 156° 20' E. longitude.