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 on the principle of powerful magnetism and feeble galvanism” which is believed to have constituted the first actual electromagnet. Michael Faraday’s researches were begun in 1831 and continued for more than twenty years. Among the most splendid of his achievements was the discovery of the phenomena and laws of magneto-electric induction, the subject of two papers communicated to the Royal Society in 1831 and 1832. Another was the magnetic rotation of the plane of polarization of light, which was effected in 1845, and for the first time established a relation between light and magnetism. This was followed at the close of the same year by the discovery of the magnetic condition of all matter, a discovery which initiated a prolonged and fruitful study of paramagnetic and diamagnetic phenomena, including magnecrystallic action and “magnetic conducting power,” now known as permeability. Throughout his researches Faraday paid special regard to the medium as the true seat of magnetic action, being to a large extent guided by his pregnant conception of “lines of force,” or of induction, which he considered to be “closed curves passing in one part of the course through the magnet to which they belong, and in the other part through space,” always tending to shorten themselves, and repelling one another when they were side by side (Exp. Res. §§ 3266–8, 3271). In 1873 James Clerk Maxwell published his classical Treatise on Electricity and Magnetism, in which Faraday’s ideas were translated into a mathematical form. Maxwell explained electric and magnetic forces, not by the action at a distance assumed by the earlier mathematicians, but by stresses in a medium filling all space, and possessing qualities like those attributed to the old luminiferous ether. In particular, he found that the calculated velocity with which it transmitted electromagnetic disturbances was equal to the observed velocity of light; hence he was led to believe, not only that his medium and the ether were one and the same, but, further, that light itself was an electromagnetic phenomenon. Since the experimental confirmation of Maxwell’s views by H. R. Hertz in 1888 (Weid. Ann., 1888, 34, 155, 551, 609; and later vols.) they have commanded universal assent, and his methods are adopted in all modern work on electricity and magnetism.

The practice of measuring magnetic induction and permeability with scientific accuracy was introduced in 1873 by H. A. Rowland, whose careful experiments led to general recognition of the fact previously ignored by nearly all investigators, that magnetic susceptibility and permeability are by no means constants (at least in the case of the ferromagnetic metals) but functions of the magnetizing force. New light was thrown upon many important details of magnetic science by J. A. Ewing’s Experimental Researches of 1885; throughout the whole of his work special attention was directed to that curious lagging action to which the author applied the now familiar term “hysteresis.” His well-known modification of Weber’s molecular theory, published in 1890, presented for the first time a simple and sufficient explanation of hysteresis and many other complexities of magnetic quality. The amazing discoveries made by J. J. Thomson in 1897 and 1898 resulted in the establishment of the electron theory, which has already effected developments of an almost revolutionary character in more than one branch of science. The application of the theory by P. Langevin to the case of molecular magnetism has been noticed above, and there can be little doubt that in the near future it will contribute to the solution of other problems which are still obscure.

See W. Gilbert, De magnete (London, 1600; trans. by P. F. Mottelay, New York, 1893, and for the Gilbert Club, London, 1900); M. Faraday, Experimental Researches in Electricity, 3 vols. (London, 1839, 1844 and 1855); W. Thomson (Lord Kelvin), Reprint of Papers on Electrostatics and Magnetism (London, 1884, containing papers on magnetic theory originally published between 1844 and 1855, with additions); J. C. Maxwell, Treatise on Electricity and Magnetism (3rd ed., Oxford, 1892); E. Mascart and J. Joubert, Leçons sur l’électricité et le magnétisme (2nd ed., Paris, 1896–1897; trans., not free from errors, by E. Atkinson, London, 1883); J. A. Ewing, Magnetic Induction in Iron and other Metals (3rd ed., London, 1900); J. J. Thomson, Recent Researches in Electricity and Magnetism (Oxford, 1893); Elements of Mathematical Theory of Electricity and Magnetism (3rd ed., Cambridge, 1904); H. du Bois, The Magnetic Circuit (trans. by E. Atkinson, London, 1896); A. Gray, Treatise on Magnetism and Electricity, vol. i. (London, 1898); J. A. Fleming, Magnets and Electric Currents (London, 1898); C. Maurain, Le magnétisme du fer (Paris, 1899; a lucid summary of the principal facts and laws, with special regard to their practical application); Rapports présentés au Congrès international de physique, vol. ii. (Paris, 1900); G. C. Foster and A. W. Porter, Treatise on Electricity and Magnetism (London, 1903); A. Winkelmann, Handbuch der Physik, vol. v. part i. (2nd ed., Leipzig, 1905; the most exhaustive compendium of magnetic science yet published, containing references to all important works and papers on every branch of the subject).

MAGNETISM, TERRESTRIAL, the science which has for its province the study of the magnetic phenomena of the earth.

§ 1. Terrestrial magnetism has a long history. Its early growth was slow, and considerable uncertainty prevails as to its earliest developments. The properties of the magnet (see ) were to some small extent known to the Greeks and Romans before the Christian era, and compasses (see ) of an elementary character seem to have been employed in Europe at least as early as the 12th century. In China and Japan compasses of a kind seem to have existed at a much earlier date, and it is even claimed that the Chinese were aware of the declination of the compass needle from the true north before the end of the 11th century. Early scientific knowledge was usually, however, a mixture of facts, very imperfectly ascertained, with philosophical imaginings. When an early writer makes a statement which to a modern reader suggests a knowledge of the declination of the compass, he may have had no such definite idea in his mind. So far as Western civilization is concerned, Columbus is usually credited with the discovery—in 1492 during his first voyage to America—that the pointing of the compass needle to the true north represents an exceptional state of matters, and that a declination in general exists, varying from place to place. The credit of these discoveries is not, however, universally conceded to Columbus. G. Hellmann 6 considers it almost certain that the departure of the needle from the true north was known in Europe before the time of Columbus. There is indirect evidence that the declination of the compass was not known in Europe in the early part of the 15th century, through the peculiarities shown by early maps believed to have been drawn solely by regard to the compass. Whether Columbus was the first to observe the declination or not, his date is at least approximately that of its discovery.

The next fundamental discovery is usually ascribed to Robert Norman, an English instrument maker. In The Newe Attractive (1581) Norman describes his discovery made some years before of the inclination or dip. The discovery was made more or less by accident, through Norman’s noticing that compass needles which were truly balanced so as to be horizontal when unmagnetized, ceased to be so after being stroked with a magnet. Norman devised a form of dip-circle, and found a value for the inclination in London which was at least not very wide of the mark.

Another fundamental discovery, that of the secular change of the declination, was made in England by Henry Gellibrand, professor of mathematics at Gresham College, who described it in his Discourse Mathematical on the Variation of the Magneticall Needle together with its Admirable Diminution lately discovered (1635). The history of this discovery affords a curious example of knowledge long delayed. William Borough, in his Discourse on the Variation of the Compas or Magneticall Needle (1581), gave for the declination at Limehouse in October 1580 the value 11° E. approximately. Observations were repeated at Limehouse, Gellibrand tells us, in 1622 by his colleague Edmund Gunter, professor of astronomy at Gresham College, who found the much smaller value 6° 13′. The difference seems to have been ascribed at first to error on Borough’s part, and no suspicion of the truth seems to have been felt until 1633, when some rough observations gave a value still lower than that found by Gunter.