Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/451

 or (neglecting )

which is the formula of Fresnel. The hypothesis of Fresnel, that a ponderable body in motion carries with it the excess of aether which it contains as compared with space free from matter, is thus seen to be transformed in Lorentz' theory into the supposition that the polarized molecules of the dielectric, like so many small condensers, increase the dielectric constant, and that it is (so to speak) this augmentation of the dielectric constant which travels with the moving matter. One evident objection to Fresnel's theory, namely, that it required the relative velocity of aether and matter to be different for light of different colours, is thus removed; for the theory of Lorentz only requires that the dielectric constant should have different values for light of different colours, and of this a satisfactory explanation is provided by the theory of dispersion.

The correctness of Lorentz' hypothesis, as opposed to that of Hertz (in which the whole of the contained aether was supposed to be transported with the moving body), was afterwards confirmed by various experiments. In 1901 R. Blondlot drove a current of air through a magnetic field, at right angles to the lines of magnetic force. The air-current was made to pass between the faces of a condenser, which were connected by a wire, so as to be at the same potential. An electromotive force would be produced in the air by its motion in the magnetic field; and, according to the theory of Hertz, this should produce an electric induction  of amount  (where  denotes the specific inductive capacity of the air, which is practically unity); so that, according to Hertz, the faces of the condenser should become charged. According to Lorentz theory, on the other hand, the electric induction is determined by the equation