The habit of considering magnetic action as the type or general case of attractive and repulsive agency, explains the early writers having spoken of Electricity as a kind of Magnetism. Thus Gilbert, in his book De Magnete (1600), has a chapter,[1] De coitione Magniticâ, primumque de Succini attractione, sive verius corporum ad Succinum applicatione. The manner in which he speaks, shows us how mysterious the fact of attraction then appeared; so that, as he says, “the magnet and amber were called in aid by philosophers as illustrations, when our sense is in the dark in abstruse inquiries, and when our reason can go no further. Gilbert speaks of these phenomena like a genuine inductive philosopher, reproving[2] those who before him had “stuffed the booksellers’ shops by copying from one another extravagant stories concerning the attraction of magnets and amber, without giving any reason from experiment.” He himself makes some important steps in the subject. He distinguishes magnetic from electric forces,[3] and is the inventor of the latter name, derived from ἤλεκτρον, electron, amber. He observes rightly, that the electric force attracts all light bodies, while the magnetic force attracts iron only; and he devises a satisfactory apparatus by which this is shown. He gives[4] a considerable list of bodies which possess the electric property; “Not only amber and agate attract small bodies, as some think, but diamond, sapphire, carbuncle, opal, amethyst, Bristol gem, beryli, crystal, glass, glass of antimony, spar of various kinds, sulphur, mastic, sealing-wax,” and other substances which he mentions. Even his speculations on the general laws of these phenomena, though vague and erroneous, as [193] at that period was unavoidable, do him no discredit when compared with the doctrines of his successors a century and a half afterwards. But such speculations belong to a succeeding part of this history.
[1] Lib. ii. cap. 2.
[2] De Magnete, p. 48.
[3] Ib. p. 52.
[4] Ib. p. 48.
In treating of these Sciences, I will speak of Electricity in the first place; although it is thus separated by the interposition of Magnetism from the succeeding subjects (Galvanism, &c.) with which its alliance seems, at first sight, the closest, and although some general notions of the laws of magnets were obtained at an earlier period than a knowledge of the corresponding relations of electric phenomena: for the theory of electric attraction and repulsion is somewhat more simple than of magnetic; was, in fact, the first obtained; and was of use in suggesting and confirming the generalization of magnetic laws.
CHAPTER 1.
Discovery of Laws of Electric Phenomena.
WE have already seen what was the state of this branch of knowledge at the beginning of the seventeenth century; and the advances made by Gilbert. We must now notice the additions which it subsequently received, and especially those which led to the discovery of general laws, and the establishment of the theory; events of this kind being those of which we have more peculiarly to trace the conditions and causes. Among the facts which we have thus especially to attend to, are the electric attractions of small bodies by amber and other substances when rubbed. Boyle, who repeated and extended the experiments of Gilbert, does not appear to have arrived at any new general notions; but Otto Guericke of Magdeburg, about the same time, made a very material step, by discovering that there was an electric force of repulsion as well as of attraction. He found that when a globe of sulphur had attracted a feather, it afterwards repelled it, till the feather had been in contact with some other body. This, when verified under a due generality of circumstances, forms a capital fact in our present subject. Hawkesbee, who wrote in 1709 (Physico-Mechanical Experiments) also observed various of the effects of attraction and repulsion upon threads hanging loosely. But the person who appears to have first fully seized the general law of these facts, is [194] Dufay, whose experiments appear in the Memoirs of the French Academy, in 1733, 1734, and 1737.[5] “I discovered,” he says, “a very simple principle, which accounts for a great part of the irregularities, and, if I may use the term, the caprices that seem to accompany most of the experiments in electricity. This principle is, that electric bodies attract all those that are not so, and repel them as soon as they are become electric by the vicinity or contact of the electric body. . . . Upon applying this principle to various experiments of electricity, any one will be surprised at the number of obscure and puzzling facts which it clears up.” By the help of this principle, he endeavors to explain several of Hawkesbee’s experiments.
[5] Priestley’s History of Electricity, p. 45, and the Memoirs quoted.