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should equal the square of the index of refraction, and that  conductors should be opaque. With his characteristic honesty, Maxwell reported that the index of refraction depends on the wave length of light, is not well measured, and that the few measurements available do not fit his theory (pp. 788-789). Furthermore, his own experiments with gold leaf indicated a transparency much greater than is consistent with his theory (p. 800). In his final evaluation (pp. 846-866) Maxwell concluded that the two approaches are empirically equivalent, but that he preferred field theory on intuitive grounds.

Subsequent developments will be presented more briefly. As Buchwald (1985, pp. 73-173) showed, Maxwell's followers eventually accepted Helmholtz's atoms of electricity as sources and current as a flow of electrons ( a term Stoney introduced) primarily because of  the difficulties displacement engendered, and because newly discovered phenomena, such as the Hall effect and the Kerr magneto-optic effect required it. Continental electrodynamics also developed. Helmholtz presented a generalized potential formula with a term, k. Assigning k a value of  -1 led to Weber's potential formula, + 1 gave Neumann's potential, while 0 reproduced Maxwell's potential. Since integration of the potential over a closed circuit yielded 0 in all cases, it could only be tested by an open circuit. Helmholtz thought Maxwell's formula was probably correct, since it yielded transverse vibrations and the correct velocity of light.

Heinrich Hertz, Helmholtz's leading student, learned electrodynamics from Maxwell's Treatise. In 1885 he began his epochal research testing open circuits as sources of electromagnetic radiation. When he showed that electromagnetic waves have a finite velocity, that they can be refracted, polarized, diffracted, reflected, and produce interference effects he precipitated a consensus within the European physics community of the correctness of Maxwell's idea of a dielectric medium transmitting electromagnetic vibrations. The acceptance, however, involved some distinct modifications. Hertz accepted the reality of fields (Hertz, 1962, p. 4) and of ether as a substratum which should be explained in mechanical terms. He did not, however, accept displacement as a change of state in the ether, for he did not know what the ether is. This lack of knowledge suggested a change in methodology. Instead of arguing to Maxwell's equations on experimental grounds, he accepted Maxwell's theory as a basis for deductions with the proviso: "Maxwell's theory is Maxwell's system of equations." (Hertz, p. 21, and p. 138). This formal approach led to discounting the physical interpretation of the dielectrical constant and the potentials. The symbol 'e' can stand for the amount of electricity contained in a system, something measurable, without speculation about what electricity really is.

Hertz's conception of the ether as a mechanical substratum included the idea that it is dragged along by the earth's motion. Lorentz introduced the idea that electrons interact with the medium as well as other electrons. An electron moving in an electric (E) and  magnetic (H) field experiences a force, F = eE + (e/c)[v x H], which can be transmitted to the ponderable body with which the electron is associated and can also be dissipated in the form of radiation. Following Stokes, Lorentz assumed a perfectly stationary ether, one perfectly transparent to matter. Because there is no equality of action and reaction this ether is non-mechanical. (See Hirosige 1969 and Nersessian, 1984) This was the penultimate step in eliminating the mechanical ether. The ultimate step, Einstein's special theory of relativity, is presumably familiar.

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