Among the substances in which great divergence 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 be 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.[1] Messrs. Romich and Nowak[2] 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[3] 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 × 104, the value obtained was lower, i.e., 5.78. Gordon, with a frequency of 1.2 × 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[4] 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[5] found K to be 2.84 when the frequency of vibration was of the order 2.5 × 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 con-
