Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/230

 mutual repulsion of these lines, or pressure laterally. Where a line of force ends on one of the spheres, its tension is exercised on the sphere: in this way, every surface-element of each sphere is pulled outwards. If the spheres were entirely removed from each other's influence, the state of stress would be uniform round each sphere, and the pulls on its surface-elements would balance, giving no resultant force on the sphere. But when the two spheres are brought into each other's presence, the unit lines of force become somewhat more crowded together on the sides of the spheres which face than on the remote sides, and thus the resultant pull on either sphere tends to draw it Loward the other, When the spheres are at distances great compared with their radii, the attraction is nearly proportional to the inverse square of the distance, which is Priestley's law.

In the following year (1838) Faraday amplified his theory of electrostatic induction, by making further use of the analogy with the induction of magnetism. Fourteen years previously Poisson had imagined an admirable model of the molecular processes which accompany magnetization; and this was now applied with very little change by Faraday to the case of induc- tion in dielectrics. "The particles of an insulating dielectric," he suggested, "whilst under induction may be compared to a series of small magnetic needles, or, more correctly still, to a series of small insulated conductors. If the space round a charged globe were filled with a mixture of an insulating dielectric, as oil of turpentine or air, and small globular conductors, as shot, the latter being at a little distance from each other so as to be insulated, then these would in their condition and action exactly resemble what I consider to be the condition and action of the particles of the insulating dielectric itself. If the globe were charged, thcso little con- ductors would all be polar; if the globe were discharged, they would all return to their normal state, to be polarized again upon the recharging of the globe."

That this explanation accounts for the phenomena of specific