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| History of Theories of Aether and Electricity | |||||||||
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Electric and magnetic Science prior to the introduction of the potentials
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this experiment that the attraction of electricity is subject to
the same laws with that of gravitation, and is therefore according
to the squares of the distances ; since it is easily demonstrated
that were the earth in the form of a shell, a body in the inside
of it would not be attracted to one side more than another ? " This brilliant inference seems to have been insufficiently studied by the scientific men of the day ; and, indeed, its author appears to have hesitated to claim for it the authority of a complete and rigorous proof. Accordingly we find that the question of the law of force was not regarded as finally settled for eighteen years afterwards. [1] By Franklin's law of the conservation of electric charge, and Priestley's law of attraction between charged bodies, electricity was raised to the position of an exact science. It is impossible to mention the names of these two friends in such a connexion without reflecting on the curious parallelism of their lives. In both men there was the same combination of intellectual boldness and power with moral earnestness and public spirit. Both of them carried on a long and tenacious struggle with the reactionary influences which dominated the English Government in the reign of George III ; and both at last, when overpowered in the conflict, reluctantly exchanged their native flag for that of the United States of America. The names of both have been held in honour by later generations, not more for their scientific discoveries than for their services to the cause of religious, intellectual, and political freedom. The most celebrated electrician of Priestley's contemporaries in London was the Hon. Henry Cavendish (b. 1731, d. 1810), whose interest in the subject was indeed hereditary, for his father, Lord Charles Cavendish, had assisted in Watson's experiments of 1747. [2] In 1771 Cavendish [3] presented to the Royal Society an " Attempt to explain some of the principal phenomena of Electricity, by means of an elastic fluid." The hypothesis adopted is that of the one-fluid theory, in much the same form as that of Aepinus. It was, as he tells us, discovered independently, although he became acquainted with Aepinus' work before the publication of his own paper. In this memoir Cavendish makes no assumption regarding the law of force between electric charges, except that it is " inversely as some less power of the distance than the cube " ; but he evidently inclines to believe in the law of the inverse square. Indeed, he shows it to be " likely, that if the electric attraction or repulsion is inversely as the square of the distance, almost all the redundant fluid in the body will be lodged close to the surface, and there pressed close together, and the rest of the body will be saturated"; which approximates closely to the discovery made four years previously by Priestley. Cavendish did, as a matter of fact, rediscover the inverse square law shortly afterwards; but, indifferent to fame, he neglected to communicate to others this and much other work of importance. The value of his researches was not realized until the middle of the nineteenth century, when William Thomson (Lord Kelvin) found in Cavendish's manuscripts the correct value for the ratio of the electric charges carried by a circular disk and a sphere of the same radius which had been placed in metallic connexion. Thomson urged that the papers should be published ; which came to pass [4] in 1879, a hundred years from the date of the great discoveries which they enshrined. It was then seen that Cavendish had anticipated his successors in several of the ideas which will presently be discussed amongst others, those of electrostatic capacity and specific inductive capacity. In the published memoir of 1771 Cavendish worked out the consequences of his fundamental hypothesis more completely than Aepinus ; and, in fact, virtually introduced the notion of electric potential, though, in the absence of any definite assumption as to the law of force, it was impossible to develop this idea to any great extent. One of the investigations with which Cavendish occupied himself was a comparison between the conducting powers of different materials for electrostatic discharges. The question had been first raised by Beccaria, who had shown [5] in 1753 that when the circuit through which a discharge is passed contains tubes of water, the shock is more powerful when the cross-section of the tubes is increased. Cavendish went into the matter much more thoroughly, and was able, in a memoir presented to the Royal Society in 1775, [6] to say : " It appears from some experiments, of which I propose shortly to lay an account before this Society, that iron wire conducts about 400 million times better than rain or distilled water - that is, the electricity meets with no more resistance in passing through a piece of iron wire 400,000,000 inches long than through a column of water of the same diameter only one inch long. Sea- water, or a solution of one part of salt in 30 of water, conducts 100 times, or a saturated solution of sea-salt about 720 times, better than rain-water." The promised account of the experiments was published in the volume edited in 1879. It appears from it that the method of testing by which Cavendish obtained these results was simply that of physiological sensation; but the figures given in the comparison of iron and sea-water are remarkably exact. While the theory of electricity was being established on a sure foundation by the great investigators of the eighteenth century, a no less remarkable development was taking place in the kindred science of magnetism, to which our attention must now be directed. The law of attraction between magnets was investigated at an earlier date than the corresponding law for electrically charged bodies. Newton, in the Principia [7] says : " The power of gravity is of a different nature from the power of magnetism. For the magnetic attraction is not as the matter attracted. Some bodies are attracted more by the magnet, others less ; most bodies not at all. The power of magnetism, in one and the same body, may be increased and diminished ; and is sometimes far stronger, for the quantity of matter, than the power of gravity ; and in receding from the magnet, decreases not in the duplicate, but almost in the triplicate proportion of the distance, as nearly as I could judge from some rude observations." The edition of the Principia which was published in 1742 by Thomas Le Seur and Francis Jacquier contains a note on this corollary, in which the correct result is obtained that the directive couple exercised on one magnet by another is proportional to the inverse cube of the distance. The first discoverer of the law of force between magnetic poles was John Michell (b. 1724, d. 1793), at that time a young Fellow of Queen's College, Cambridge, [8] who in 1750 published A Treatise of Artificial Magnets ; in which is shown an easy and expeditious method of making them superior to the lest natural ones. In this he states the principles of magnetic theory as follows [9] : " Wherever any Magnetism is found, whether in the Magnet itself, or any piece of Iron, etc., excited by the Magnet, there are always found two Poles, which are generally called North and South ; and the North Pole of one Magnet always attracts the South Pole, and repels the North Pole of another: and vice versa" This is of course adopted from Gilbert. "Each Pole attracts or repels exactly equally, at equal distances, in every direction." This, it may be observed, overthrows the theory of vortices, with which it is irreconcilable. " The Magnetical Attraction and Repulsion are exactly equal to each other." This, obvious though it may seem to us, was really a most important advance, for, as he remarks, " Most people, who have mention'd any thing relating to this property of the Magnet, have agreed, not only that the Attraction and Repulsion of Magnets are not equal to each other, but that also, they do not observe the same rule of increase and decrease." " The Attraction and Repulsion of Magnets decreases, as the Squares of the distances from the respective poles increase." This great discovery, which is the basis of the mathematical theory of Magnetism, was deduced partly from his own observations, and partly from those of previous investigators (e.g. Dr. Brook Taylor and P. Muschenbroek), who, as he observes, had made accurate experiments, but had failed to take into account all the considerations necessary for a sound theoretical discussion of them. After Michell the law of the inverse square was maintained by Tobias Mayer [10] of Gottingen (b. 1723, d. 1762), better known as the author of Lunar Tables which were long in use ; and by the celebrated mathematician, Johann Heinrich Lambert [11] (b. 1728, d. 1777). The promulgation of the one-fluid theory of electricity, in the middle of the eighteenth century, naturally led to attempts to construct a similar theory of magnetism ; this was effected in 1759 by Aepinus [12], who supposed the "poles "to be places at which a magnetic fluid was present in amount exceeding or falling short of the normal quantity. The permanence of magnets was accounted for by supposing the fluid to be entangled in their pores, so as to be with difficulty displaced. The particles of the fluid were assumed to repel each other, and to attract the particles of iron and steel ; but, as Aepinus saw, in order to satisfactorily explain magnetic phenomena it was necessary to assume also a mutual repulsion among the material particles of the magnet. Subsequently two imponderable magnetic fluids, to which the names boreal and austral were assigned, were postulated by the Hollander Anton Brugmans (5. 1732, d. 1789) and by Wilcke. These fluids were supposed to have properties of mutual attraction and repulsion similar to those possessed by vitreous and resinous electricity. The writer who next claims our attention for his services both to magnetism and to electricity is the French physicist, Charles Augustin Coulomb [13] (ft. 1736, d. 1806). By aid of the torsion-balance, which was independently invented by Michell and himself, he verified in 1785 Priestley's fundamental law that the repulsive force between two small globes charged with the same kind of electricity is in the inverse ratio of the square of the distance of their centres. In the second memoir he extended this law to the attraction of opposite electricities. Coulomb did not accept the one-fluid theory of Franklin, Aepinus, and Cavendish, but preferred a rival hypothesis which had been proposed in 1759 by Robert Symmer. [14] " My notion," said Symmer, " is that the operations of electricity do not depend upon one single positive power, according to the opinion generally received; but upon two distinct, positive, and active powers, which, by contrasting, and, as it were, counteracting each other, produce the various phenomena of electricity ; and that, when a body is said to be positively electrified, it is not simply that it is possessed of a larger share of electric matter than in a natural state ; nor, when it is said to be negatively electrified, of a less ; but that, in the former case, it is possessed of a larger portion of one of those active powers, and in the latter, of a larger portion of the other ; while a body in its natural state remains unelectrified, from an equal ballance of those two powers within it." Coulomb developed this idea : " Whatever be the cause of electricity," he says, [15] " we can explain all the phenomena by [1] In 1769 Dr. John Robison (b. 1739, d. 1805), of Edinburgh, endeavoured to determine the law of force by direct experiment, and found it to be tbat of the inverse 2'06 th power of the distance. [2] Phil. Trans, xlv, p. 67 (1750). [3] Phil. Trans. Ixi, p. 584 (1771). [4] The Electrical Researches of the Hon. Henry Cavendish, edited by J. Clerk Maxwell, 1879. [5] G. B. Beccaria, Dell'elettricismo artificiale e naturale, Turin. 1753, p. 113. [6] Phil. Trans. Ixvi (1776), p. 196. [7] Book iii, Prop, vi, cor. 5. [8] Michell had taken his degree only two years previously. Later in life he was on terms of friendship with Priestley, Cavendish, and William Herschel ; it was he who taught Herschel the art of grinding mirrors for telescopes. The plan of determining the density of the earth, which was carried out by Cavendish in 1798, and is generally known as the " Cavendish Experiment," was due to Michell. Michell was the first inventor of the torsion-balance ; he also made many valuable contributions to Astronomy. In 1767 he became Rector of Thornhill, Yorks, and lived there until his death. [9] Loc. cit., p. 17. [10] Noticed in Gottinger Gelehrter Anzeiger, 1760 : cf. Aepinus, Nov. Comm. Acad. Petrop., 1768, and Mayer's Opera Inedita, herausg. von G. C. Lichtenberg. [11] Histoire de L'Acad. de Berlin, 1766, pp. 22, 49. [12] In the Tentamen, to which reference has already been made. [13] Coulomb's First, Second, and Third Memoirs appear in Memoires de 1'Acad., 1785 ; the Fourth in 1786, the Fifth in 1787, the Sixth in 1788, and the Seventh in 1789. [14] Phil. Trans, li (1759), p. 371. [15] Sixth Memoir, p. 561. |