126 Mr. H. H. Poole on the Dielectric Constant and 



leakage-current with time. The lowest reading plotted in 

 the figure represents a current density of about 10 ~ 5 micro- 

 ampere, or iso^yo of the greatest current observed with the 

 galvanometer. For these small gradients the value of the 

 specific resistance found from the curve is about 1*36 x 10 15 . 



Fig. 3. 



x Iff 











/\ A 









/-^ 





A 



x ! 





















A 











y 



/ 



























0.2 



04 



There was some evidence that, at still smaller gradients, a 

 further increase in the specific resistance occurred, but the 

 readings were rendered most uncertain by soakage currents. 

 On earthing the tinfoil disk which had been connected to 

 the pressure source, a small reverse current w r as always 

 noticeable. A reverse current about 3 x 10" 4 microampere 

 per sq. cm. was observed immediately after the mica had 

 been subjected to a gradient of 0*536 megavolt per cm. 

 (which caused a current 16*8 X 10~ 4 ). This reverse current 

 died away rapidly at first, but even after several days a 

 current about 4 X 10" 7 microampere per sq. cm. was observed. 



