392 Prof. W. M. Thornton on the Polarization of 



It was found further that there was no permanent residual 

 electrification in the dielectrics examined, showing that the 

 polarization, however long continued and slow in action, was 

 quasi-elastic in type. 



When testing electric cables the charging current is some- 

 times measured after one minute's electrification. With the 

 usual insulators this is only in part a true conduction current, 

 the rest being the second stage of polarization. What the 

 latter would be in any condenser can be calculated from the 

 curves of rate of change of apparent dielectric constant given 

 later, and in this way the true conductivity found. 



2. Experiments on Quartz. 



The first systematic examination of the influence of time 

 of charge upon the values obtained for dielectric con- 

 stants was made by Boltzmann*, and by Romich and 

 Nowakf under his direction. They obtained for quartz, for 

 example, a value greater than 1000 in steady fields. All 

 subsequent observers recognizing these results have specified 

 the duration of charge. The effect was dealt with by 

 J. Hopkinson J in an important series of papers, and the 

 recent work of Beaulard§ upon the dielectric constant of 

 w r ater at high frequencies is a good illustration of it. With 

 the exception of Komich and Nowak's experiments, observa- 

 tions of the influence of steady fields have been mostly made 

 on the charge and discharge currents of condensers, and in 

 their interpretation the relative importance of polarization 

 and conduction must be separated. By a conduction current 

 is meant one produced by the passage of free electrons or 

 ions, and this takes place much more freely when the 

 dielectric is in direct contact with the terminal plates between 

 which the pressure is maintained. A very complete resume of 

 work on dielectrics up to 1904 is given by L. Graetz in Band 

 iv. of Winkelmann's Handbuch der Physik. A paper by 

 Von Schweidler, in Annalen der Physik ||, deals with more 

 recent work and gives a full bibliography If. 



In the present experiments the first substance examined 

 was quartz cut parallel to the optic axis and formed into an 



* Wien. Ber. Ixvi., lxvii. (1872-3). 

 t Wien. Ber. lxx. (1874). 

 t Original Papers, vol. ii. nos. 18, 19, 27. 

 § Science Abstracts, 1905 to 1908. 

 il Ann. der Pfojs. xxiv. p. 711 (1907). 



■f] See also H. A. Wilson, Roy. Soc. Proc. A. lxxxi. p. 409, " On Electro- 

 static Induction through Solid Insulators." 



