Benjamin Franklin

An interest in electrical experiments spread from du Fay to other members of the Court circle of Louis 15th. 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 2 currents of a fluid, “very subtle and inflammable”. This he supposed to be present in all bodies always.[31]

When all electric is excited by friction, part of this fluid escapes from its pores. It forms an effluent stream. This loss is repaired by an afluent stream of the same fluid entering the body from outside.

Light bodies in the vicinity are caught in one or other of these streams and are attracted or repelled from the excited electric.

Nollet’s theory was in great vogue for some time.

But 6 or 7 years after its first publication, he came across a work purporting to be a French translation of Benjamin Franklin’s book printed originally in England.

• It described his experiments made at Philadelphia.

Franklin tells us in his Autobiography:

“Nollet could not at first believe that such a work came from America. He said it must have been fabricated by his enemies at Paris to decry his system. After he was assured that Franklin did do it at Philadelphia, he published a volume of letters, chiefly addressed to me, defending his theory. He denied the verity of my experiments, and of the positions deduced from them.”

In 1745, Pieter van Musschenbroek (b. 1692, d. 1761), Professor at Leyden, tried to preserve electric charges from the decay which was observed when the charged bodies were surrounded by air. He surrounded 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. 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.[32]

Nollet named it the Leyden phial. Shortly after its discovery, a London apothecary named William Watson (b. 1715, d. 1787)[33] noticed that in the experiment, the observer feels the shock “in no other parts of his body but his arms and breast”. From this 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. 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[34] 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.

Watson wrote: “I have shewn that:

• electricity is the effect of a very subtil and elastic fluid, occupying all bodies in contact with the terraqueous globe
• everywhere, 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
• 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, Dr. Spence had lately arrived in America from Scotland. He was showing in Boston some electrical experiments.

Among his audience was a 40 year old Benjamin Franklin of Philadelphia (b. 1706, d. 1790), already known as a leading citizen of the English colonies in America.

Spence’s experiments “were,” writes Franklin,[35] “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,[36] 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 do not need to admit that 2 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.”

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.