290 Scientific Proceedings, Royal Dublin Society. 



and outer diameter 1.1 cms.; ab a carbon rod, on which P was threaded; 

 CC larger carbon rods ; A and B tightly wrapped bands of copper wire ; V a 

 hot-wire voltmeter used as a milliammeter ; and M a megger. 



M was first connected to E and F, with A and B about 3*5 cms. apart, 

 and a current was passed through ab. The reading of M was infinity until 

 ab became more heated ; when dull-red the resistance was 20 megohms, and 

 at incandescence 15,000 ohms. On switching off the current, the resistance 

 gradually increased as cooling proceeded, reaching infinity again. 



The distance between A and It was then reduced to about l - 5 millimetres, 

 and E and F connected through V to alternating-current mains of voltage 

 300 and frequency 51 - 5. 



"When ab was incandescent, V indicated 80 milliamperes, so that the 



resistance of the porcelain between A and B was p—- — = 4000 ohms nearly. 



O'Oo 



On switching off the heating current, the reading of V fell quickly to zero. 



In order to test whether any effect was produced by gaseous or ionic 

 conduction, E and F were connected to two points, s and t, one millimetre 

 apart, near the surface of P, and afterwards to two similar points, m and n, 

 close to the carbon when ab was incandescent, but in both cases V indicated 

 zero. 



The first important investigation of the resistance of porcelain and other 

 substances was made by Foussereau in 1894. His sample was in the form of 

 a small tube, closed at one end, and mercury was used for the inside and 

 outside contact surfaces. The results 1 obtained by him for a sample of 

 porcelain were given for a range of temperature from 50° C. to 210° 0. At the 

 former temperature the resistance of the sample was 2150 x 10 6 megohms 

 and 6510 megohms at the latter. That is, the resistance at 50° C. was 

 330,000 times the value of that at 210° C. 



In 1909 Pirani and Siemens investigated the change of resistance of 

 porcelain in the form of tubes for different temperatures ranging from 

 613° C. to 900° C. for one sample, and from 727° C. to 1292 c C. for another. 

 Among the values 2 given for the first sample were l - 098 megohms for 613° C., 

 and 68,700 ohms for 900° C. For the second sample 100,000 ohms for 

 727° C, and 3,400 ohms for 1292° C. were given. 



Dietrich, in 1910, made a similar investigation upon certain insulators, 

 including porcelain, and obtained results 3 for a sample of the latter material 

 at temperatures ranging from 20° C. to 189° C. At the former temperature 



1 Ann. de Chim. et de Phys., 6 me Ser., vol. 5, 1885, p. 371, Table 29. 

 3 Zeitschr. Elektrochem. 15. 1909. p. 969. Tables 5 and 6. 

 3 Phys. Zeit., 11 January, 1910, Table, p. 188. 



