Mica, Ebonite and Gutta-percha. 163 



stance of the glass. It would, therefore, seem undesirable to 

 obtain capacity by thinness of wall. 



Internal resistance of gutta-percha. — Two sheets of gutta- 

 percha were employed, their thickness being 2*79™°^. Tinfoil 

 was cemented on them with a solution of gutta-percha, and 

 the results were fairly constant as seen below, where the 

 actual fiojures obtained are o:iven : 



No. 1 



6,030,000 



fi 



Feb. 19th 



Hyg. 18 



" 2 



6,270,000 



u 



u u 



ii u 



" 1 



5,240,000 



a 



'' 24th 



" • 27 



'' 2 



6,640,000 



a 



a a 



a a 



6,050,000 " 

 Calculating this for a surface of one square centimeter and 

 a thickness of one millimeter, we obtain, 18,500,000 12. 



Internal resistance of ehonite. — Three plates of this material 

 were used, one having a thickness of 2-29"'^ ; the other two 

 were only •5°'°' thick. The coatings were. made of a metallic 

 powder backed by tinfoil, all of which probably did not insure 

 the best possible contact. 



Reducing the results to the standard of one square centi- 

 meter of surface and a thickness of one millimeter, we have 

 No. 1 76,400,000 O Hvg. 32° 

 " 2 45,200,000 " " " 

 " 3 45,000,000 " " " 



55,000,000 



Internal resistance of mica (muscovite, beautiful large sheets). 

 — The thickness of No. 1 was •122'°'" ; Nos. 2 and 3 had a 

 thickness of -32™™. The coatings were made in the same way 

 as with ebonite, deducing to an area of one square centi- 

 meter and a thickness of 1™°", I obtained for 



No. 1 100,000,000 O Hyg. 32° 

 " 2 190,000,000 " '*• " 



133,000,000 " 



From this it would seem that the internal resistance of mica 

 is somewhat greater than that of ebonite, though, as will be 

 shown later on, its surface resistance is much smaller. 



Surface resistance of plate glass (window glass, commercial). 

 — This proved to be unexpectedly regular, in spite of some 

 variations in the hygrometric condition of the air. Two plates 

 were employed, each having a length of 23*7"" between the 

 terminals. The actual figures obtained on three different occa- 

 sions are as follows : 



