Superphysics
Chapter 2e

Franklin's law of the conservation of electric charge

6 minutes  • 1084 words

Regarding the physical nature of electricity, Benjamin Franklin held the same ideas as his contemporaries.

He pictured it as an elastic fluid, consisting of particles extremely subtle that it can permeate matter, even the densest metals, with such ease and freedom as not to receive any perceptible resistance.

But he did not believe that all electrical repulsions are caused by the diffusion of effluvia from the excited electric to the body acted on.

• The tickling sensation 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 practically ended with Franklin.

It bears a suggestive resemblance to that which nearly a century later was introduced by Faraday.

Both Franklin and Faradady explained electrical phenomena without introducing action at a distance. That idea supposes that something which forms an essential part of the electrified system is present at the spot where any electric action takes place.

In the older theory, this something was identified with the electric fluid itself.

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 from the explanation of the Leyden jar, a matter which is discussed in his third letter to Collinson, of date September 1, 1747.

In charging the jar, a quantity of electricity is taken away from one side of the glass, by means of the coating in contact with it. 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[38] 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[39] 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.

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.[40]

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.

He writes,[41]: “Thus, 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. The difficulty was eventually removed by Aepinus.

Franklin believed in the power of electricity to act at a distance. But he did not abandon the doctrine of effluvia:

“The form of the electrical atmosphere, 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.

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 near electrified bodies. A current of dry air does not destroy electric attractions and repulsions.[45]

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 is in all bodies as well as electrical fire. Perhaps they are 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 the attention which it deserved from European philosophers. Watson generously tried to make his merits known[47]. He inserted some of Franklin’s letters in one of his own papers communicated to the Royal Society.

Franklin’s discoveries were printed in England. The naturalist Buffon was so much impressed that he secured the issue of a French translation of the work.

This publication gave such offence to Nollet.

The success of a plan proposed by Franklin for drawing lightning from the clouds soon engaged public attention everywhere. 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."[48]

The theory of effluvia was finally overthrown and replaced by action at a distance by the labours of Francis Ulrich Theodore Aepinus[49] (b. 1724, d. 1802), one of Franklin’s continental followers.