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XXII. On the Measurement of Dielectric Constants by means 

 of Hertz Oscillations. By Ernst Lecher*. 



THERE can be no doubt that the agreement demanded by 

 Maxwell's theory between the dielectric constant and 

 the square of the refractive index does not hold good in the 

 majority of cases ; and therefore, if w T e still wish to regard 

 Maxwell's theory as true, disturbing secondary phenomena 

 arise which alter the value of the dielectric constant as or- 

 dinarily determined. In the case of perfect insulators — 

 whose number is, however, very small — a satisfactory agree- 

 ment has already been found ; but with less perfect insulators 

 the discrepancies are very considerable. Now Schiller f has 

 shown that for some substances the dielectric constants, 

 measured with a very short time of charge, fulfil Maxwell's 

 condition more nearly than in experiments with statical 

 electricity. Moreover, the experiments and theoretical con- 

 siderations of Cohn and Arons % seem to point to an increase 

 of dielectric constants with time of charge. The idea there- 

 fore suggests itself that the agreement with Maxwell's theory 

 only holds for the case of an extremely short time of charge. 



Starting with similar ideas, J. J. Thomson § has measured 

 some dielectric constants by the aid of Hertz oscillations. 

 More recently, without any knowledge of Thomson's work, 

 I have carried out almost identical experiments. As, how- 

 ever, my results differ from those of Thomson, I venture to 

 communicate them. 



I found, to wit, that in the case of four substances which 

 I investigated, the dielectric constant calculated from the 

 capacity in the simple manner about to be described not only 

 did not sink, but actually became greater, when Hertz 

 oscillations were used. 



Although I have used the term " dielectric constant," this 

 is really not quite accurate. I have only compared capacities 

 with each other, in which either air or the substance to be 

 investigated was placed between the plates. Consequently 

 in my results, in addition to dielectric action, we have con- 

 duction, residual charge, and other unknown errors creeping 



* Translated from Wiedemann's Annalen, vol. xlii. p. 142 (Jan. 1891), 

 by James L. Howard. 



t Schiller, Pog& Ann. clii. p. 535, 1874. 



X E. Cohn and L. Arons, Wied. Ann. xxviii. p. 454, 1886, and xxxiii. 

 p. 13, 1888. 



§ J. J. Thomson, Proc. Roy. Soc. xlvi. p. 292, 1889. 



