308 



Scientific Proceedings, Royal Dublin Society. 



Sample H in Fig. 6 shows a higher insulation resistance than sample G 

 for temperatures above 100° C, and less for temperatures below this value. 



The following table was derived from the curves of Fig. 6 for a few ordinary 

 temperatures, and the results may be regarded as rough representations of the 

 insulation resistance in megohms per cm. cube of 'the different samples of 

 porcelain experimented with. For the lower temperatures the lines were 

 continued onward a little farther than that given in the figure : — 



Table 13. 



Professor Miles "Walker in a table 1 of insulating materials gave the specific 

 insulating resistance of porcelain at 25° C. as ranging from 10 6 to 10 9 megohms 

 per cm. cube for different samples. Excepting G and H, Table 13 shows 

 that the values found are of the same order as those given by him, but the 

 range for the samples used, including G and H, is not quite so extensive. 



Curtis in his paper, referred to on page 297, gives 300 x 10° megohms 

 per cm. cube for the value of unglazed porcelain at a temperature of 22° C, 

 which corresponds to the value found for A and B. 



By continuing the curves of Fig. 6 in the direction of higher temperatures 

 the insulation resistance for temperatures beyond that experimented with 

 may be roughly determined. 



The results of the present investigation are : — 



1. A further demonstration that, whatever the quality, thickness, or form 

 of the porcelain as illustrated by the eight samples used, the relation of 

 Basch and Hinrichsen between insulation resistance and absolute temperature 

 is approximately true. 



An important use of this fact is that by finding the insulation resistance 

 for two representative temperatures, the value for other temperatures may 

 be approximately determined. 



1 Specification and Design of Dynamo-Electric Machinery. — Miles Walker, p. 176, 

 Table 9, 1915 edition. 



