Electric And Magnetic Science Prior To The Introduction Of The Potentials

Descartes tried to explain the magnetic discoveries of Peregrinus and Gilbert through the vortex-hypothesis [1].

These have raised magnetism to the rank of a separate science by the mid-17th century.

• It has since been considerably advanced by Gilbert.

For 2,000 years, the attractive power of amber had been regarded as a virtue peculiar to amber, or possessed by at most one or two others.

Gilbert proved[2] this view to be wrong.

• He showed that the same effects are induced by friction in many bodies like:
  • glass
  • sulphur
  • sealing-wax
  • various precious stones.

A force which was manifested by so many different kinds of matter seemed to need a name of its own.

• Accordingly, Gilbert gave to it the name ’electric'.

Between the magnetic and electric forces, Gilbert remarked many distinctions.

The lodestone requires 110 stimulus of friction such as is needed to stir glass and sulphur into activity.

• The lodestone attracts only magnetizable substances, whereas electrified bodies attract everything.

The magnetic attraction between two bodies is not affected by interposing a sheet of paper, or a linen cloth, or by immersing the bodies in water.

Whereas the electric attraction is readily destroyed by screens.

Lastly, the magnetic force tends to arrange bodies in definite orientations; while the electric force merely tends to heap them together in shapeless clusters.

These facts appeared to Gilbert to indicate that electric phenomena are due to something of a material nature, which under the influence of friction is liberated from the glass or amber in which under ordinary circumstances it is imprisoned.

In support of this view he adduced evidence from other quarters.

Being a physician, he was well acquainted with the doctrine that the human body contains various humours or kinds of moisture-phlegm, blood, choler, and melancholy,—which, as they predominated, were supposed to determine the temper of mind.

When he observed that electrifiable bodies were almost all hard and transparent, and therefore (according to the ideas of that time) formed by the consolidation of watery liquids, he concluded that the common menstruum of these liquids must be a particular kind of humour, to the possession of which the electrical properties of bodies were to be referred.

Friction might be supposed to warn or otherwise excite or liberate the hunour, which would then issue from the body as an effluvium and form an atmosphere around it.

The effluvium must, he remarked, be very attenuated, for its emission cannot be detected by the senses.

The existence of an atmosphere of effluvia round every electrified body have been inferred, according to Gilbert’s ideas, from the single fact of electric attraction.

For he believed that matter cannot act where it is not.

Hence if a body acts on all surrounding objects without appearing to touch them, something must have proceeded out of it unseen.

The whole phenomenon appeared to him to be analogous to the attraction which is exercised by the earth on falling bodies.

For in the latter case he conceived of the atmospheric air as the effluvium by which the earth draws all things downwards to itself.

Gilbert’s theory of electrical emanations commended itself generally to 17th-century natural philosophers such as:

• Niccolo Cabeo (b. 1585, d. 1650), an Italian Jesuit who was perhaps the first to observe that electrified bodies repel as well as attract
• the English royalist exile, Sir Kenelm Digby (b. 1603, d. 1665)
• the celebrated Robert Boyle (b. 1627, d. 1691).

There were some differences of opinion as to the manner in which the effluvia acted on the small bodies and set them in motion towards the excited electric.

Gilbert himself had supposed the emanations to have an inherent tendency to reunion with the parent body.

Digby likened their return to the condensation of a vapour by cooling; and other writers pictured the effluvia as forming vortices round the attracted bodies in the Cartesian fashion.

There is a well-known allusion to Gilbert’s hypothesis in Newton’s Opticks.[3]

“Let him also tell me, how an electrick body can by friction emit an exhalation so rare and subtle,[4] and yet so potent, as by its emission to cause no sensible diminution of the weight of the electrick body, and to be expanded through a sphere, whose diameter is above 2 feet.

Yet to be able to agitate and carry up leaf copper, or leaf gold, at a distance of above a foot from the electrick body?”

It is surprising that the Newtonian doctrine of gravitation should not have proved a severe blow to the emanation theory of electricity.

But Gilbert’s doctrine was now so firmly established as to be unshaken by the overthrow of the analogy by which it had been originally justified.

It was, however, modified in one particular about the beginning of the 18th century.

In order to account for the fact that electrics are not perceptibly wasted away by excitement, tho earlier writers had supposed all the emanations to return ultimately to the body which had emitted them.

But the corpuscular theory of light accustomed philosophers to the idea of emissions so subtle as to cause no perceptible loss; and after the time of Newton the doctrine of the return of the electric effluvia gradually lost credit.