Superphysics
Chapter 2g

# The one-fluid theory of electricity

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After Michell the law of the inverse square was maintained by Tobias Mayer[65] of Göttingen (b. 1723, d. 1762), better known as the author of Lunar Tables which were long in use; and by the celebrated mathematician, Johann Heinrich Lambert[66] (b. 1728, d. 1777)

The one-fluid theory of electricity was promulgated in the mid-18th century.

It naturally led to attempts to construct a similar theory of magnetism. This was effected in 1759 by Aepinus,[67] who supposed the “poles” to be places at which a magnetic fluid was present in amount exceeding or falling short of the normal quantity.

The permanence of magnets was accounted for by supposing the fluid to be entangled in their pores, so as to be with difficulty displaced. The particles of the fluid were assumed to repel each other, and to attract the particles of iron and steel; but, as Aepinus saw, in order to satisfactorily explain magnetic phenomena it was necessary to assume also a mutual repulsion among the material particles of the magnet.

Subsequently two imponderable magnetic fluids, to which the names boreal and austral were assigned, were postulated by the Hollander Anton Brugmans (b. 1732, d. 1789) and by Wilcke. These fluids were supposed to have properties of mutual attraction and repulsion similar to those possessed by vitreous and resinous electricity.

The writer who next claims our attention for his services both to magnetism and to electricity is the French physicist, Charles Augustin Coulomb[68] (b. 1736, d. 1806). By aid of the torsion-balance, which was independently invented by Michell and himself, he verified in 1785 Priestley’s fundamental law that the repulsive force between two small globes charged with the same kind of electricity is in the inverse ratio of the square of the distance of their centres. In the second memoir he extended this law to the attraction of opposite electricities.

Coulomb did not accept the one-fluid theory of Franklin, Aepinus, and Cavendish, but preferred a rival hypothesis which had been proposed in 1759 by Robert Symmer.[69]

Symmer says: “My notion is that the operations of electricity do not depend on one single positive power. Instead, it depends on 2 distinct, positive, and active contrasting powers. These counteract each other and produce the various phenomena of electricity.

When a body is positively electrified, it has more of those active powers.

When a body is negatively electrified, it has more of the other powers.

When a body is in its natural state, it remains unelectrified, from an equal balance of those two powers within it.

Coulomb developed this idea:

“Whatever be the cause of electricity,” he says,[70] “we can explain all the phenomena by supposing that there are two electric fluids, the parts of the same fluid repelling each other according to the inverse square of the distance, and attracting the parts of the other fluid according to the same inverse square law.”

“The supposition of two fluids,” he adds, “is moreover in accord with all those discoveries of modern chemists and physicists, which have made known to a various pairs of gases whose elasticity is destroyed by their admixture in certain proportions—an effect which could pot take place without something equivalent to a repulsion between the parts of the same gas, which is the cause of its elasticity, and an attraction between the parts of different gases, which accounts for the loss of elasticity on combination.”

According to this two-fluid theory, the “natural fluid” contained in all matter can be decomposed, under the influence of an electric field, into equal quantities of vitreous and resinous electricity, which, if the matter be conducting, can then fly to the surface of the body.

The abeyance of the characteristic properties of the opposite electricities when in combination was sometimes further compared to the neutrality manifested by . the compound of an acid and an alkali.

The publication of Coulomb’s views led to some controversy between the partisans of the one-fluid and two-fluid theories. The latter was soon generally adopted in France.

But it was stoutly opposed in Holland by Van Marum and in Italy by Volta.

The chief difference is that, in the two-fluid theory, both the electric fluids are movable within the substance of a solid conductor.

In the one-fluid theory the actual electric fluid is mobile, but the particles of the conductor are fixed.

The dispute could therefore be settled only by a determination of the actual motion of electricity in discharges; and this was beyond the reach of experiment.

In his Fourth Memoir, Coulomb showed that electricity in equilibrium is confined to the surface of conductors, and does not penetrate to their interior substance.

In the Sixth Memoir[71] he virtually establishes the result that the electric force near a conductor is proportional to the surface-density of electrification,

Since the overthrow of the doctrine of electric eflluvia by Aepinus, the aim of electricians had been to establish their science upon the foundation of a law of action at a distance, resembling that which had led to such triumphs in Celestial Mechanics.

When the law first stated by Priestley was at length decisively established by Coulomb, its simplicity and beauty gave rise to a general feeling of complete trust in it as the best attainable conception of electrostatic phenomena. The result was that attention was almost exclusively focused on action-at-a-distance theories, until the time, long afterwards, when Faraday led natural philosophers back to the right: path.

Coulomb rendered great services to magnetic theory. By simple mechanical reasoning in 1777, he completed the overthrow of the hypothesis of vortices.[72]

His second Memoir[73] confirmed Michell’s law – the particles of the magnetic fluids attract or repel each other with forces proportional to the inverse square of the distance.

Coulomb, however, went beyond this. He accounted for the fact that the two magnetic fluids, unlike the two electric fluids, cannot be obtained separately.

When a magnet is broken into 2 pieces, one containing its north and the other its south pole, each piece is an independent magnet possessing two poles of its own. It is impossible to obtain a north or south pole in a state of isolation.

Coulomb explained this by supposing[74] that the magnetic fluids are permanently imprisoned within the molecules of magnetic bodies, so as to be incapable of crossing from one molecule to the next.

Each molecule therefore under all circumstances contains as much of the boreal as of the austral fluid, and magnetization consists simply in a separation of the two fluids to opposite ends of each molecule.

Such a hypothesis evidently accounts for the impossibility of separating the two fluids to opposite ends of a body of finite size. The same idea, here introduced for the first time, has since been applied with success in other departments of electrical philosophy.

The mathematical development of electric and magnetic theory was scarcely begun at the close of the 18th century. Many erroneous notions were still widely entertained.

Report[75] which was presented to the French Academy in 1800, it was assumed that the mutual repulsion of the particles of electricity on the surface of a body is balanced by the resistance of the surrounding air. The electric force outside a charged conductor was long confused with a supposed additional pressure in the atmosphere.