Du Fay and the Frenchby Edmund Taylor Whittaker
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 Abbé 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.
When all electric is excited by friction, part of this fluid escapes from its pores, forming an effluent stream; and this loss is repaired by an afluent 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 (b. 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 Cunaeus, 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.
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 (b. 1715, d. 1787) 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 afflux and efflux; and there can indeed be little doubt that the Abbé’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 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, “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, 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 illuminated 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 fluid, consisting of “particles extremely subtle, 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 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 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.
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, “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 repulsion of resinously electrified bodies became known to him, it caused him considerable perplexity. As we shall sec, 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, “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 teaspoon, 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.” HC 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.
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, “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, 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 élève and immediate disciple."
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 (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 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: 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 con 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 bad been more or less indefinitely observed by many previous writers, and specially studied a short time previously by John Canton (b. 1718, d. 1772) and by Wilcket—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 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 (lid 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. “May we not infer,” he says, “from 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.
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. In 1771 Cavendish 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  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 hall been first raised by Beccaria, who had shown 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, 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, 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 t poles was John Michell (b. 1724, d. 1793), at that time a young Fellow of Queen’s College, Cambridge, who in 1750 published A Treatise of Artificial Magnets; in which is shown an easy and expeditious method of making them superior to the best natural ones. In this he states the principles of magnetic theory as follows:—
“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 obscrvations, and partly from those of previous investigators (e.g. Dr. Brook Taylor and P. Musschenbroek[errata 1]), 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,