| Whittaker | |||||||||
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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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nated with a star or globule, while the point which is near the
outer coating is illuminated with a pencil of rays; which
suggested to Franklin that the electric fluid, going from the
inside to the outside of the jar, enters at the former point and
issues from the latter. And yet again, in some cases the flame
of a wax taper is blown away from a brass ball which is
discharging vitreous electricity, and towards one which is
discharging resinous electricity. But Franklin remarks that
the interpretation of these observations is somewhat conjectural,
and that whether vitreous or resinous electricity is the actual
electric fluid is not certainly known. Regarding the physical nature of electricity, Franklin held much the same ideas as his contemporaries ; he pictured it as an elastic [1] fluid, consisting of " particles extremely subtile, since it can permeate common matter, even the densest metals, with such ease and freedom as not to receive any perceptible resistance." He departed, however, to some extent from the conceptions of his predecessors, who were accustomed to ascribe all electrical repulsions to the diffusion of effluvia from the excited electric to the body acted on ; so that the tickling sensation which is experienced when a charged body is brought near to the human face was attributed to a direct action of the effluvia on the skin. This doctrine, which, as we shall see, practically ended with Franklin, bears a suggestive resemblance to that which nearly a century later was introduced by Faraday ; both explained electrical phenomena without introducing action at a distance, by supposing that something which forms an essential part of the electrified system is present at the spot where any electric action takes place ; but in the older theory this something was identified with the electric fluid itself, while in the modern view it is identified with a state of stress in the aether. In the interval between the fall of one school and the rise of the other, the theory of action at a distance was dominant. The germs of the last-mentioned theory may be found in Franklin's own writings. It originated in connexion with the explanation of the Leyden jar, a matter which is discussed in his third letter to Collinson, of date September 1st, 1747. In charging the jar, he says, a quantity of electricity is taken away from one side of the glass, by means of the coating in contact with it, and an equal quantity is communicated to the other side, by means of the other coating. The glass itself he supposes to be impermeable to the electric fluid, so that the deficiency on the one side can permanently coexist with the redundancy on the other, so long as the two sides are not connected with each other ; but when a connexion is set up, the distribution of fluid is equalized through the body of the experimenter, who receives a shock. Compelled by this theory of the jar to regard glass as impenetrable to electric effluvia, Franklin was nevertheless well aware [2] that the interposition of a glass plate between an electrified body and the objects of its attraction does not shield the latter from the attractive influence. He was thus driven to suppose [3] that the surface of the glass which is nearest the excited body is directly affected, and is able to exert an influence through the glass on the opposite surface ; the latter surface, which thus receives a kind of secondary or derived excitement, is responsible for the electric effects beyond it. This idea harmonized admirably with the phenomena of the jar ; for it was now possible to hold that the excess of electricity on the inner face exercises a repellent action through the substance of the glass, and so causes a deficiency on the outer faces by driving away the electricity from it. [4] Franklin had thus arrived at what was really a theory of action at a distance between the particles of the electric fluid ; and this he was able to support by other experiments. " Thus," he writes, [5] " the stream of a fountain, naturally dense and continual, when electrified, will separate and spread in the form of a brush, every drop endeavouring to recede from every other drop.' In order to account for the attraction between oppositely charged bodies, in one of which there is an excess of electricity as compared with ordinary matter, and in the other an excess of ordinary matter as compared with electricity, he assumed that " though the particles of electrical matter do repel each other, they are strongly attracted by all other matter " ; so that " common matter is as a kind of spunge to the electrical fluid." These repellent and attractive powers he assigned only to the actual (vitreous) electric fluid; and when later on the mutual repidsion of resinously electrified bodies became known to him, [6] it caused him considerable perplexity. [7] As we shall see, the difficulty was eventually removed by Aepinus. In spite of his belief in the power of electricity to act at a distance, Franklin did not abandon the doctrine of effluvia. "The form of the electrical atmosphere," he says, [8] "is that of the body it surrounds. This shape may be rendered visible in a still air, by raising a smoke from dry rosin dropt into a hot tea- spoon under the electrified body, which will be attracted, and spread itself equally on all sides, covering and concealing the body, And this form it takes, because it is attracted by all parts of the surface of the body, though it cannot enter the substance already replete. Without this attraction, it would not remain round the body, but dissipate in the air." He observed, however, that electrical effluvia do not seem to affect, or be affected by, the air ; since it is possible to breathe freely in the neighbourhood of electrified bodies ; and moreover a current of dry air does not destroy electric attractions and repulsions. [9] Regarding the suspected identity of electricity with the matter of heat, as to which Nollet had taken the affirmative position, Franklin expressed no opinion. " Common fire," he writes, [10] " is in all bodies, more or less, as well as electrical fire. Perhaps they may be different modifications of the same element ; or they may be different elements. The latter is by some suspected. If they are different things, yet they may and do subsist together in the same body." Franklin's work did not at first receive from European philosophers the attention which it deserved ; although Watson generously endeavoured to make the colonial writer's merits known, [11] and inserted some of Franklin's letters in one of his own papers communicated to the Royal Society. But an account of Franklin's discoveries, which had been printed in England, happened to fall into the hands of the naturalist Buffon, who was so much impressed that he secured the issue of a French translation of the work ; and it was this publication which, as we have seen, gave such offence to Nollet. The success of a plan proposed by Franklin for drawing lightning from the clouds soon engaged public attention everywhere; and in a short time the triumph of the one-fluid theory of electricity, as the hypothesis of Watson and Franklin is generally called, was complete. Nollet, who was obdurate, "lived to see himself the last of his sect, except Monsieur B - of Paris, his eleve and immediate disciple." [12] The theory of effluvia was finally overthrown, and replaced by that of action at a distance, by the labours of one of Franklin's continental followers, Francis Ulrich Theodore Aepinus [13](b. 1724, d. 1802). The doctrine that glass is impermeable to electricity, which had formed the basis of Franklin's theory of the Leyden phial, was generalized by Aepinus [14] and his co-worker Johann Karl Wilcke (b. 1732, d. 1796) into the law that all non-conductors are impermeable to the electric fluid. That this applies even to air they proved by constructing a machine analogous to the Leyden jar, in which, however, air took the place of glass as the medium between two oppositely charged surfaces. The success of this experiment led Aepinus to deny altogether the existence of electric effluvia surrounding charged bodies : [15] a position which he regarded as strengthened by Franklin's observation, that the electric field in the neighbourhood of an excited body is not destroyed when the adjacent air is blown away. The electric fluid must therefore be supposed not to extend beyond the excited bodies themselves. The experiment of Gray, to which we have already referred, showed that it does not penetrate far into their substance; and thus it became necessary to suppose that the electric fluid, in its state of rest, is confined to thin layers on the surfaces of the excited bodies. This being granted, the attractions and repulsions observed between the bodies compel us to believe that electricity acts at a distance across the intervening air. Since two vitreously charged bodies repel each other, the force between two particles of the electric fluid must (on Franklin's one-fluid theory, which Aepinus adopted) be repulsive : and since there is 'an attraction between oppositely charged bodies, the force between electricity and ordinary matter must be attractive. These assumptions had been made, as we have seen, by Franklin; but in order to account for the repulsion between two resinously charged bodies, Aepinus introduced a new supposition - namely, that the particles of ordinary matter repel each other. This, at first, startled his contemporaries; but, as he pointed out, the "unelectrified" matter with which we are acquainted is really matter saturated with its natural quantity of the electric fluid, and the forces due to the matter and fluid balance each other ; or perhaps, as he suggested, a slight want of equality between these forces might give, as a residual, the force of gravitation. Assuming that the attractive and repellent forces increase as " the distance between the acting charges decreases, Aepinus applied his theory to explain a phenomenon which had been more or less indefinitely observed by many previous writers, and specially studied a short time previously by John Canton [16] (b. 1718, d. 1772) and by Wilcke [17] - namely, that if a conductor is brought into the neighbourhood of an excited body without actually touching it, the remoter portion of the conductor acquires an electric charge of the same kind as that of the excited body, while the nearer portion acquires a charge of the opposite kind. This effect, which is known as the induction of electric charges, had been explained by Canton himself and by Franklin [18] in terms of the theory of electric effluvia. Aepinus showed that it followed naturally from the theory of action at a distance, by taking into account the mobility of the electric fluid in conductors ; and by discussing different cases, so far as was possible with the means at his command, he laid the foundations of the mathematical theory of electrostatics. Aepinus did not succeed in determining the law according to which the force between two electric charges varies with the distance between them ; and the honour of having first accomplished this belongs to Joseph Priestley (b. 1733, d. 1804), the discoverer of oxygen. Priestley, who was a friend of Franklin's, had been informed by the latter that he had found cork balls to be wholly unaffected by the electricity of a metal cup within which they were held ; and Franklin desired Priestley to repeat and ascertain the fact. Accordingly, on December 21st, 1766, Priestley instituted experiments, which showed that, when a hollow metallic vessel is electrified, there is no charge on the inner surface (except near the opening), and no electric force in the air inside. From this he at once drew the correct conclusion, which was published in 1767. [19] " May we not infer," he says, "from [1] i.c., repulsive of its own particles. [2] New Experiments, 1750, 28. t Hid., 1750, 34. [3] Ibid., 1750, 34. [4] Ibid., 1750, 32. Letter v. [5] Letter v. [6] He refers to it in his Paper read to the Royal Society, December 18, 1755. [7] Cf. letters xxxvii and xxxviii, dated 1761 and 1762. [8] New Experiment , 1750, 15. [9] Letter vii, 1751. [10] Letter v. [11] Phil. Trans, xlvii, p. 202. Watson agreed with Nollet in rejecting Franklin's theory of the impermeability of glass. [12] Franklin's Autobiography. [13] This philosopher's surname had been hellenized from its original form by one of his ancestors, a distinguished theologian. [14] F. V. T. Aepinus Tentamen Theoriae Electricitatis et Magnetismi : St. Petersburg, 1759. [15] This was also mainteined about the same time by Giacomo Battista Beccaria of Turin (b. 1716, d. 1781;. [16] Phil. Trans, xlviii (1753), p. 350. [17] Disputatio physica experimentalis de electricitatibus contrariis : Rostock, 1757. [18] In his paper read to the Royal Society on Dec. 18th, 1755. [18] Priestley, The History and Present State of Electricity, with Original Experiments ; London, 1767: page 732. That electrical attraction follows the law of the inverse square had been suspected by Daniel Bernoulli in 1760: Cf. Soci's Experiments, Acta Helvetica, iv, p. 214. |