Page:On the Determination of the Index of Refraction of Glass for the the Electric Ray.djvu/2

294 from Cauchy’s formula, which is admittedly faulty when applied to rays below the visible spectrum. It would therefore be of interest to be able to measure directly the index for long electric waves, and, compare it with the value of K for rapidly alternating electric fields, the periodicity of which is preferably of the same order as that of the electric waves for which the index is determined.

Among the substances in which great divergence is exhibited between the values of K and μ2, glass may be taken as typical. In the very carefully conducted series of experiments by Hopkinson the value of K (later results) was found to he 6·61 for light flint and 9·81 for extra dense flint glass. He found no variation of K with the time of charge, which varied from 1/4 to 1/20,000 part of a second. Messrs. Romich and Nowak found the value to be 7·5 for alternation of field of about once in a second, while for steady fields they obtained the abnormally high value of 159. Schiller found K for plate glass to be 6·34, with a frequency of alternation of 25 in a second. With a higher frequency of about 1·2 X 104, the value obtained was lower, i.e., 5·78. Gordon, with a frequency of 1·2 X 104, obtained 3·24 as K for common glass.

From the experiments of Schiller it would appear that the value of K for glass diminished with the increase of frequency of alternation of the field.

Rubens and Arons compared the velocities of propagation of electro-magnetic action through air and glass, and obtained the ratio of the velocities or μ = 2·33. The deduced value of K would therefore be 5·43. M. Blondlot found K to be 2·84 when the frequency of vibration was of the order 2·5 X 107. Professor J. J. Thomson found the specific inductive capacity of glass to be smaller under rapidly changing fields than in steady ones. He deduced the value of K by measuring the lengths of wave emitted by a parallel plate condenser with air and glass as dielectrics. The value for glass was found to be 2·7.

On the other hand, Lecher found that the dielectric constant rose with the frequency of vibration. Thus for plate glass—