Allen — The Electrical Resistance of Porcelain. 



299 



These workers in the case of porcelain used the results of Foussereau, 

 already referred to on p. 290, and showed that this formula applied to that 

 material. 



Pirani and Siemens, whose results are referred to on p. 290, demonstrated 

 from them that this formula represented also the change of resistance of 

 porcelain with higher temperatures ranging from 600° C. to 1300° C. An 

 equivalent formula was also given by Dietrich in his paper referred to on 

 p. 290. 



The following Tables represent the results obtained for the eight samples 

 of porcelain named on p. 291. The thickness of each sample is given at the 

 top of each table. Specific insulation resistance in megohms per cm. cube is 

 represented by o- ; t is the mean temperature of the mercury contact-surfaces 

 reckoned from zero in degrees centigrade ; and T is the absolute temperature, 

 namely, t + 273. In each case the porcelain is of the glazed variety, 

 excepting the inner surface of the E, which, though unglazed, was very 

 smooth. 



Table 4. 



Sample A. 



Thickness 0'7 cm. 



This, in common with the other Tables, shows the enormous decrease of 

 insulation resistance with increase of temperature. 



3 c 2 



