Conductivity of some Dielectrics. 497 



earthed guard-ring. This method (which would be inap- 

 plicable with mica owing to its softness) worked excellently 

 with glass, the insulation formed by the dividing line being- 

 found to be quite satisfactory. 



The results plotted in fig. 5 were obtained with a specimen 



Fiff. 5. 



0\129 mm. thick, the test area being about 0'93 sq. cm. 

 The small circles represent readings with increasing fields, 

 the large ones readings when the field was being subse- 

 quently reduced. It will be noticed that at L5° C, when 

 the conduction current is comparatively small, the former 

 ones are consistently higher than the latter. This may be 



