Allkn — The Electrical Resistance of Porcelain. 309 



2. For temperatures below about 250° C. the insulation resistance of some 

 samples of porcelain is practically independent of the voltage, and nearly so 

 in the case of the others tested. Sample B was tested up to 800 volts at 

 temp. 180° C, and its insulation resistance found to be approximately 

 independent of the voltage. Others were tested up to 200 volts. 



3. The enormous differences between the values of the insulation 

 resistance for the different varieties of porcelain are shown by Table 13 

 and Fig. 6. 



Thus Portland China and Eoyal Worcester porcelain as represented 

 respectively by G and H have a very much higher insulation resistance than 

 the other varieties of porcelain, especially that used for the low tension 

 porcelain insulators A and B. 



D, E, and F, representing some kinds of German porcelain, have their 

 lines in Fig. 6 roughly grouped together, and these throughout the tests 

 showed more dielectric absorption than the other samples. 



In Fig. 6 the line of A and B is nearly parallel to that of H, so that the 

 ratio of the insulation resistance of the latter to that of A or B is almost 

 independent of the temperature for the range given, and the .value of this 

 ratio is about 140. 



The insulation resistance of the 1862 porcelain insulator represented by 

 line C is higher than that of either of the recent types, namely A and B. 



Thus by means of a diagram such as Fig. 6 one may determine the 

 probable identity of a sample of unknown porcelain by experimentally 

 determining its insulation resistance at two suitable temperatures. 



SCIENT. PROO. R.D.S., VOL. XV., NO. XXVII. 3 E 



