82 The Ohio Journal of Science [Vol. XVI, No. 3, 



in applying a potential difference to porcelain bodies which 

 were varied in temperature and determined the dielectric 

 strength of the porcelain at different temperatures. They 

 made a large number of such bodies, of different composition 

 and of a form adapted for use in an electrically heated furnace. 

 They measured the potentials required to break down such 

 insulators. 



In general when such a body breaks down under electric 

 stress a mechanical puncture results and the body ceases to 

 become an effective insulator when again subject to high 

 potentials. Messrs. Henderson and Weimar found that when a 

 temperature of 300° C. was reached, they had difficulty in 

 building up a potential to a value where a definite and sharp 

 break indicated a puncture of the dielectric. Instead of 

 obtaining such a definite value a rather indefinite one was 

 obtained in which a phenomenon somewhat similar to break 

 down occurred. They discovered also that when these insula- 

 tors were cooled to room temperatures that their insulating 

 properties had not been impaired in the slightest. This 

 indicated that instead of producing a mechanical break down 

 of the material such as was attained at lower temperatures, the 

 failure to insulate was due to a change in conductivity of the 

 bodies. 



Soon after this, experiments were reported by A. A. 

 Sommerville of Cornell University and W. W. Stifler of the 

 University of Illinois* on the resistance of similar bodies at 

 high temperatures. The methods employed were the usual 

 Wheatstone bridge methods for determining resistance in 

 which a small e. m. f. was used. 



Fleming, in his discussion of cable insulation, points out 

 that such a method is open to serious objection on account 

 ■of the polarization of the dielectric. 



The effect of applying an e. m. f. develops a back e. m. f. 

 and the condition for establishing a bridge balance becomes a 

 function of the true resistance of the arm containing the speci- 

 men and the e. m. f. of polarization as well. The whole question 

 of the resistance of such bodies is an open one. The trans- 

 mission of current through such bodies has been regarded at 



1^* Physical Review, April, 1911, p. 429. 



