Magnetism

Gilbert discovered that a piece of iron was not attracted if red hot, but it regained its normal properties on cooling. He scouts the idea that the peculiarity of iron, as distinguished from other metals, is due to its being cold, as had been alleged, "as if, forsooth, cold were cause of attraction, or iron were much colder than lead, which neither follows the lodestone nor leans toward it. But this is sorry trifling, no better than old wives' gossip." He found that bars of iron could be magnetized by hammering them when they were held to point north and south, particularly if the hammering was carried out as the iron cooled from red heat.

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other books of De Magnete, Gilbert treats fully of the direc tive force, or "verticity" of the lodestone ; of magnetic declination (the divergence of a magnet needle from the true north and south direction) and its variation; and of magnetic dip. He describes forms of the compass. Gilbert's main discovery—that the earth itself was a great magnet—should perhaps be called a magnificent theory which explained the then known facts of terrestrial mag netism with beautiful and coherent simplicity. With a terrella of lodestone and a magnetized needle, the results of the theory could be illustrated in a striking and satisfying manner. The conception of the earth as a magnet swept away the older views as to why a magnet pointed to the north. "The common herd of philosophers, in search of the causes of magnetic movements, called in causes remote and far away. Martinus Cortenius dreamt of an attractive magnetic point beyond the heavens. . . . Petrus Peregrinus holds that direction has its rise at the celestial poles. . . . So ever has been the wont of mankind ; homely things are vile ; things from abroad and things afar are dear to them and the object of longing." To account for magnetic actions, Gilbert favoured an effluvium theory, since matter cannot act where it is not. Such a theory was not devoid of use for descriptive purposes, and the small ex tent to which Gilbert indulges in fruitless speculation in attempts at explanation is noteworthy.

It is rather remarkable that Francis Bacon did scant justice to the work of one who practised the method which he preached. "Gilbert has attempted to raise a general system upon the mag net," he says, "endeavouring to build a ship out of materials not sufficient to make the rowing pins of a boat." Indeed, quite gen erally, Gilbert's work suffered neglect. Possibly this was partly due to his cosmological conclusions, in which magnetism played an overimportant role—the Copernican theory was little assisted by the intended support. De Magnete, in any case, could hardly have been popular among Gilbert's contemporary scientists, the strictures were too severe, but in the main, the neglect was due to the fact that Gilbert's outlook was considerably in advance of his time. Actually his book is an epitome of what was known about magnetism not only in his time, but for practically zoo years after wards. Typical of the solemn errors which persisted long after Gilbert's time is a statement in van Helmont's De Magnetica (1621) : "The lodestone onely by the affriction of Garlick, amits its verticity, and neglects the pole, conserving to itself, in the meantime, its peculiar forme, materiell constitution, and all other dependent proprieties. The reason, because Garlick is the lode

stone's proper Opium, and by it that spirituell sensation in the magnet is consopited and layd asleep." Michell, Coulomb and Poisson.—Although there was much experimental activity in the century following Gilbert, there was little progress in the particular sciences of electricity and mag netism. In 1698, Edmund Halley went on an Atlantic voyage (the expedition was equipped by the Government) on which he made valuable observations on the variation of declination. In magnetism proper no material advance was made until experiments were undertaken with a view to finding quantitative relations. The credit for the discovery of the law of force between magnetic poles is probably due to John Michell (1724-93) who, shortly after taking his degree at Cambridge, published A Treatise of Arti ficial Magnets (1750) in which he states the principles of magnetic theory. Michell was the first inventor of the torsion balance which he utilized in his experiments on magnetic forces. He found that in a. magnet "each pole attracts or repels exactly equally, at i equal distances, in every direction," which was quite at variance with prevailing ideas, and was incompatible with the theory of vortices. From his own observations and those of previous in vestigators, whose theok etical treatment of their results had not been sound, Michell made the important deduction on which the mathematical theory of magnetism is based : "The Attraction and Repulsion of Magnets decreases, as the squares of the distances from the respective poles increase." This law was later main tained by the German astronomer J. Tobias Mayer, and the Al satian mathematician J. Heinrich Lambert. John Robison 1805) also showed that the inverse square law held closely, using magnets in the form of thin rods with spherical end pieces—the localization of polarity simplifying the calculations. The direct experimental determination of the forces between magnetic poles is a matter of greater difficulty than that of the forces between electric charges, for the "magnetic charge" is not generally con fined to a small region, and even considering the resultant poles, between two magnets there are always four forces in action. The inverse square law cannot be said to have been established experimentally with satisfactory precision until C. A. Coulomb (1736-1806) carried out his classical investigations (1785). He used long thin steel rods symmetrically magnetized. In one series of experiments he suspended one rod in his torsion balance and arranged a second vertically, so the forces between the remote poles were negligible. Allowance was made for the magnetic field of the earth. The force at different distances from a magnetic pole was also calculated from the time of vibration of a small needle.