due to the action of forces at a distance, as such action would necessarily be rectilinear; and finally, in 1837, the discovery of specific inductive capacity.
This discovery as made by Faraday consisted in determining the division of an electric charge between two equal conductors when one was surrounded by air and the other by some other insulating medium. The apparatus used consisted of two brass balls 2.33 inches in diameter and exactly alike mounted upon insulating supports concentrically inside of two similar hollow brass spheres 3.57 inches internal diameter. The inner balls were carefully insulated from the outer hollow conductors, and the space between could be left filled with air or could be filled with some insulating solid or liquid. The outer hollow spheres were joined to earth.
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Thus in the diagram in Figure 1 the inner spheres are indicated by A and B, the outer hollow spheres by C and D. C and D are joined to earth. If, now, A and B are connected and charged, then separated and the hollow spheres removed from around them, they are found to have equal charges.
Faraday found that when the space between A and C was half filled with sulphur and A and B were connected and charged as before and the outer spheres and the sulphur were removed A was found to have 2.24 times as great a charge as B. The experiment was repeated with glass, shellac and other substances instead of sulphur, and it was found that in every case A took a greater charge than B, but that the magnitude of the charge depended upon the insulating material between A and C.
Faraday believed that the so-called charges upon A and B were merely manifestations of some condition known as induction which had been produced in the medium between the inner and outer spheres, though why this condition should persist after the medium was removed he does not say. In Article 1174 of Experimental Researches he says:
