266 



SILSBEE AND HONAMAN: CONDUCTIVITY MEASUREMENTS 



TABLE I 

 Resistivity of Insulating Materials 



Material 



Fused silica 



Best porcelain tested 



Typical mica plug 



Average three aviation porcelains. 

 Average automobile porcelain. . . . 



p at 500° C. 



340 X lO« 



80 X 10* 



70 X 10^ 



40 X 10' 



0.8 X io« 



ments on different specimens of material which are supposed to 

 be identical show wide variations in resistivity amounting in 

 some cases to a factor of 10. This fact tends to indicate that the 

 conduction is due to a considerable extent to the presence of 

 small amounts of impurities which may vary greatly in amount 

 without appreciably affecting the composition of the material 

 as a whole. 



It appears from the above data that the A. C. method developed 

 is very practical and convenient for comparative measurements 

 on samples of this character, but that there is a very wide field 

 of investigation concerning the phenomenon of polarization and 

 much interesting work may be done in developing theories as 

 to the precise mechanism by which conduction is carried on in 

 this class of materials. 



