Dielectrics in a Steady Field of torce. 



393 



ellipsoid 2 cm. long and *2 cm. diameter. It was very care- 

 fully cleaned, dried, and suspended by a quartz fibre about 

 *001 cm. diameter and 20 cm. long between flat brass plates 

 6 cm. apart, contained in a vessel dried under vacuum and 

 by phosphoric anhydride for several weeks. The leak between 

 the plates would therefore be exceedingly small, and with a 

 potential-difference of 480 volts between the plates the ob- 

 served periods were the same in dry air and in vacuo. In 

 order to obtain more than two figure accuracy by this method 

 it is necessary to take the mean of at least five observations of 

 the period. In the first few minutes this was as a rule changing 

 quickly, and it was only in the later stages of the polarization 

 that several readings could be taken. 



Particulars of the form and dimensions of the specimens 

 used in this paper are given in that dealing with alternating 

 fields*. 



The free period of the quartz ellipsoid was G3*3 seconds. 

 The following table gives the observed periods in fields of 

 various strength for the first 20 minutes of polarization. 



Table I. 

 F is the intensity of the external polarizing field in Electrostatic Units. 



F. 



10 s. 



15 s. 



30 s. 



1 in. 



2 m. 



3 in. 



4 m. 



L 

 5 in. 



in. 



7 in. 



8 m. 10 m. 1 15 m. 20 m. 



•087 





515 



465 



427 



400 



380 



366 



355 



34-5 



340 



33-6 



330 



31-8 



31*3 



•116 





400 



365 



32-7 



300 



28-7 



28-0 



272 



266 



200 



25-6 



25-2 



240 



23-6 



•179 





350 



30-0 



24-5 215 



20-5 20-0 



195 



190 



18-7 



186 



181 



17-0 



160 



•284 



320 





21-5 



18-0 



160 



150 14-2 



135 



132 



130 



125 



11-7 



105 



102 



•404 



211 





150 



13-0 



116 



107 10-5 



9-7 



9-2 



90 



8-7 



8-2 



7*5 



72 



•500 



151 



... 



130 



11-3 



100 



9-0 8-5 



80 



7*5 



7-2 



7-0 



67 



6-2 



60 



•747 



11-6 





100 



8-75 



73 



&5 5 - 75 



5 35 



50 



4-75 



473 



435 



405 



40 



The time at the head of each column is that at which the 

 observation was made ; the figures in the column are the 

 measured period in seconds. After each set of readings 

 the ellipsoid was allowed to stand about twelve hours to 

 depolarize. On reversing the polarity of the plates after 

 about one minute's electrification the ellipsoid turned through 

 180 degrees and followed any subsequent reversal. In five 



* Row Soc. Proc. loc. cit. 



