232 Mr. T. Gray on the Electric Resistance 



These results show a variation considerably smaller than 

 would be given by the logarithmic curve derived from the 

 results at low temperatures. So far as my experiments have 

 gone, it seems as if in the case of porcelain a slight increase 

 of resistance were produced by the first heating, and a dimi- 

 nution by subsequent heatings. This result, however, requires 

 further verification; possibly heating to a certain temperature 

 produces increase, and heating to a higher temperature dimi- 

 nution, of resistance. 



I made also some rough determinations of the specific induc- 

 tive capacity of the glass and porcelain before and after the 

 heatings to which they were subjected, and also some attempts 

 to measure it at various temperatures while the glass was 

 being heated and cooled . Although my experimental arrange- 

 ments* were not such as to give absolute results with accu- 

 racy, those obtained at low temperatures may fairly be com- 

 pared with one another. 



As these determinations were merely preliminary, I think it 

 unnecessary to enter into particulars regarding them, and con- 

 tent myself with referring to a more extended series of expe- 

 riments which I am carrying out with improved apparatus and 

 experimental arrangements, and an account of which I hope 

 to publish before long. So far as my present results go, they 

 show that the combined effect of heating and cooling is to 

 diminish the specific inductive capacity of glass. Some expe- 

 riments made on a Thomson electrometer-jar seemed to show 

 a temporary or quasi-elastic increase of specific inductive capa- 

 city with heating and diminution by cooling ; but this result 

 requires verification by more numerous experiments. 



The following table of results, which show the effect of heat- 

 ing a test-tube to 240° C. in a sand-bath and allowing it to 

 cool slowly, illustrates what has been stated above. The num- 

 bers in the first column are proportional to the specific induc- 

 tive capacity of the test-tube; and those in the second column 

 give the temperature : — 



51 12° 



Heated to 240° and allowed to cool slowly, 



46 23° 



55 57 



50 40 



These figures show a diminution of about 13 or 14 per cent. 



* The arrangement was one of a Wippe or rapid commutator, with a 

 ballistic galvanometer through which the condenser was discharged. The 

 speed of the Wippe, however, was not sufficiently great. 



