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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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He found, in fact, that when gold-leaf which had been
electrified by contact with excited glass was brought near to an
excited piece of copal,[1] an attraction was manifested between
them. " I had expected," he writes, " quite the opposite effect,
since, according to my reasoning, the copal and gold-leaf, which
were both electrified, should have repelled each other."
Proceeding with his experiments he found that the gold-leaf,
when electrified and repelled by glass, was attracted by all
electrified resinous substances, and that when repelled by the
latter it was attracted by the glass. " We see, then," he continues,
" that there are two electricities of a totally different nature -
namely, that of transparent solids, such as glass, crystal, &c.,
and that of bituminous or resinous bodies, such as amber, copal,
sealing-wax, &c. Each of them repels bodies which have
contracted an electricity of the same nature as its own, and
attracts those whose electricity is of the contrary nature. We
see even that bodies which are not themselves electrics can
acquire either of these electricities, and that then their effects
are similar to those of the bodies which have communicated it
to them." To the two kinds of electricity whose existence was thus demonstrated, du Fay gave the names vitreous and resinous, by which they have ever since been known. An interest in electrical experiments seems to have spread from du Fay to other members of the Court circle of Louis XV ; and from 1745 onwards the Memoirs of the Academy contain a series of papers on the subject by the Abbe Jean-Antoine Nollet {b. 1700, d. 1770), afterwards preceptor in natural philosophy to the Royal Family. Nollet attributed electric phenomena to the movement in opposite directions of two currents of a fluid, " very subtle and inflammable," which he supposed to be present in all bodies under all circumstances. [2] When an electric is excited by friction, part of this fluid escapes from its pores, forming an effluent stream; and this loss is repaired by an affluent stream of the same fluid entering the body from outside. Light bodies in the vicinity, being caught in one or other of these streams, are attracted or repelled from the excited electric. Nollet's theory was in great vogue for some time ; but six or seven years after its first publication, its author came across a work purporting to be a French translation of a book printed originally in England, describing experiments said to have been made at Philadelphia, in America, by one Benjamin Franklin. "He could not at first believe," as Franklin tells us in his Autobiography, " that such a work came from America, and said it must have been fabricated by his enemies at Paris to decry his system. Afterwards, having been assured that there really existed such a person as Franklin at Philadelphia, which he had doubted, he wrote and published a volume of letters, chiefly addressed to me, defending his theory, and denying the verity of my experiments, and of the positions deduced from them." We must now trace the events which led up to the discovery which so perturbed Nollet. In 1745 Pieter van Musschenbroek (6. 1692, d. 1761), Professor at Leyden, attempted to find a method of preserving electric charges from the decay which was observed when the charged bodies were surrounded by air. With this purpose he tried the effect of surrounding a charged mass of water by an envelope of some non-conductor, e.g., glass. In one of his experiments, a phial of water was suspended from a gun- barrel by a wire let down a few inches into the water through the cork; and the gun-barrel, suspended on silk lines, was applied so near an excited glass globe that some metallic fringes inserted into the gun-barrel touched the globe in motion. Under these circumstances a friend named Cimaeus, who happened to grasp the phial with one hand, and touch the gun- barrel with the other, received a violent shock ; and it became evident that a method of accumulating or intensifying the electric power had been discovered. [3] Shortly after the discovery of the Leyden phial, as it was named by Nollet, had become known in England, a London apothecary named William Watson (6. 1715, d. 1787) [4] noticed that when the experiment is performed in this fashion the observer feels the shock " in no other parts of his body but his arms and breast " ; whence he inferred that in the act of discharge there is a transference of something which takes the shortest or best- conducting path between the gun-barrel and the phial. This idea of transference seemed to him to bear some similarity to Nollet's doctrine of afflux and efflux; and there can indeed be little doubt that the Abbe's hypothesis, though totally false in itself, furnished some of the ideas from which Watson, with the guidance of experiment, constructed a correct theory. In a memoir [5] read to the Royal Society in October, 1746, he propounded the doctrine that electrical actions are due to the presence of an " electrical aether," which in the charging or discharging of a Leyden jar is transferred, but is not created or destroyed. The excitation of an electric, according to this view, consists not in the evoking of anything from within the electric itself without compensation, but in the accumulation of a surplus of electrical aether by the electric at the expense of some other body, whose stock is accordingly depleted. All bodies were supposed to possess a certain natural store, which could be drawn upon for this purpose. " I have shewn," wrote Watson, " that electricity is the effect of a very subtil and elastic fluid, occupying all bodies in contact with the terraqueous globe ; and that every-where, in its natural state, it is of the same degree of density ; and that glass and other bodies, which we denominate electrics per se, have the power, by certain known operations, of taking this fluid from one body, and conveying it to another, in a quantity sufficient to be obvious to all our senses; and that, under certain circumstances, it was possible to render the electricity in some bodies more rare than it naturally is, and, by communicating this to other bodies, to give them an additional quantity, and make their electricity more dense." In the same year in which Watson's theory was proposed, a certain Dr. Spence, who had lately arrived in America from Scotland, was showing in Boston some electrical experiments. Among his audience was a man who already at forty years of age was recognized as one of the leading citizens of the English colonies in America, Benjamin Franklin of Philadelphia (b. 1706, d. 1790). Spence's experiments " were," writes Franklin, [6] " imperfectly performed, as he was not very expert ; but, being on a subject quite new to me, they equally surprised and pleased me." Soon after this, the "Library Company" of Philadelphia (an institution founded by Franklin himself) received from Mr. Peter Collinson of London a present of a glass tube, with some account of its use. In a letter written to Collinson on July 11th, 1747, [7] Franklin described experiments made with this tube, and certain deductions which he had drawn from them. If one person A, standing on wax so that electricity cannot pass from him to the ground, rubs the tube, and if another person B, likewise standing on wax, passes his knuckle along near the glass so as to receive its electricity, then both A and B will be capable of giving a spark to a third person C standing on the floor; that is, they will be electrified. If, however, A and B touch each other, either during or after the rubbing, they will not be electrified. This observation suggested to Franklin the same hypothesis that (unknown to him) had been propounded a few months previously by Watson : namely, that electricity is an element present in a certain proportion in all matter in its normal condition ; so that, before the rubbing, each of the persons A, B, and C has an equal share. The effect of the rubbing is to transfer some of A's electricity to the glass, whence it is transferred to B. Thus A has a deficiency and B a superfluity of electricity ; and if either of them approaches C, who has the normal amount, the distribution will be equalized by a spark. If, however, A and B are in contact, electricity flows between them so as to re-establish the original equality, and neither is then electrified with reference to C. Thus electricity is not created by rubbing the glass, but only transferred to the glass from the rubber, so that the rubber loses exactly as much as the glass gains ; the, total quantity of electricity in any insulated system is invariable. This assertion is usually known as the principle of conservation of electric charge. The condition of A and B in the experiment can evidently be expressed by plus and minus signs : A having a deficiency - e and B a superfluity + e of electricity. Franklin, at the commencement of his own experiments, was not acquainted with du Fay's discoveries ; but it is evident that the electric fluid of Franklin is identical with the vitreous electricity of du Fay, and that du Fay's resinous electricity is, in Franklin's theory, merely the deficiency of a stock of vitreous electricity supposed to be possessed naturally by all ponderable bodies. In Franklin's theory we are spared the necessity for admitting that two quasi-material bodies can by their union annihilate each other, as vitreous and resinous electricity were supposed to do. Some curiosity will naturally be felt as to the considerations which induced Franklin to attribute the positive character to vitreous rather than to resinous electricity. They seem to have been founded on a comparison of the brush discharges from conductors charged with the two electricities; when the electricity was resinous, the discharge was observed to spread over the surface of the opposite conductor " as if it flowed from it." Again, if a Leyden jar whose inner coating is electrified vitreously is discharged silently by a conductor, of whose pointed ends one is near the knob and the other near the outer coating, the point which is near the knob is seen in the dark to be illumi- [1] A hard transparent resin, used in the preparation of varnish. [2] Cf. Nollet' s Recherches, 1749, p. 245. [3] The discovery was made independently in the same year by Ewald Georg von Kleist, Dean of Kumrain. [4] Watson afterwards rose to eminence in the medical profession, and was knighted. [5] Phil. Trans, xliv., p. 718. It may here he noted that it was Watson who improved the phial by coating it nearly to the top, both inside and outside, with tinfoil. [6] Franklin's Autobiography. [7] Franklin's New Experiments and Observations on Electricity, letter ii. |