Allen — The Electrical Resistance of Porcelain. 



293 



0-005 microfarad for the lowest temperatures. A voltage of about 120, or 

 lower, as required, was used for the galvanometer method, and the voltage of 

 the megger at rated speed was 500. 



The range of the megger was 10,000 ohms to 40 megohms, and could only 

 be used for the higher temperatures. 



In the case of the galvanometer it was found that 0*363 volt operating 

 through a resistance of 10,000 ohms gave a deflection of 152 divisions. 



From this the formula 



E 

 B = 449 — megohms 



was derived; E being the volts across the sample, D the deflection of the 

 galvanometer, and B the resistance of the sample in megohms. 



By using E as 100, and the lowest value of I) as 4, a resistance as high as 

 about 100 megohms could be determined by the galvanometer method. 



The range of the leakage method depends upon the capacity and time of 

 leakage. Assuming a capacity of 0'005 microfarad, an initial voltage of 102, 

 and a voltage of 100 after 10 minutes' leakage, the corresponding resistance 

 is given by 



B 



t 



°V, 



megohms = 



600 



2-3 x 0-005 x log 1-02 



which is roughly equal to about 6'5 x 10 6 megohms. 



The determination of a high specific insulation resistance will depend upon 

 the effective area and thickness of the sample as well as upon the range of the 

 method. 



From the area and thickness of each sample the formula for determining 

 a, the value of the insulation resistance in megohms per cm. cube, in terms 

 of B, was found to be as follows : — 



