162 Rood — Electrical Resistance of Glass, Quartz, 



these were used to build up a large set of high resistances sim- 

 ilar to those described in my article already alluded to. All 

 this was done twice, and during these operations, the resist- 

 ances of the insulating substances under experiment were 

 determined, as far as possible, immediately. In the case of 

 surface conduction, tinfoil was cemented on the ends of the 

 strip and fringed with a line of metallic powder. For internal 

 conduction, metallic surfaces were fastened to opposite sides of 

 the plate, which was provided with a guard-rim of tinfoil, 

 cemented with a solution of gum arable and glycerine all 

 around its edges, and grounded to prevent surface electricity 

 from reaching its opposite side ; a precaution which in the 

 case of quartz and ebonite is not necessary, quartz giving the 

 same result with and without the guard- rim, and the surface 

 conduction of ebonite in dry weather is again still smaller than 

 that of quartz. 



In order to compare together the results obtained, those for 

 surface conduction are reduced to the calculated resistance 

 that would be offered by one surface of one square centimeter 

 between the terminals. For internal resistance, the calculated 

 resistance of coatings one square centimeter in area, with a 

 thickness of one millimeter of substance between them, is 

 given. A set of one hundred dry cells was employed in each 

 case ; its electromotive force was 150 volts. The striking dis- 

 tance of the electrometer was one millimeter, except in the 

 case of very high resistances, where it was reduced to one-half 

 or one-quarter of a millimeter. 



Internal resistance of quartz. — The thickness of the plate 

 was I'Tl™"", the metallic coatings, each, one square centimeter 

 in area. Calculating the resistance for a thickness of 1™"", I 

 obtained, on 



Feb. 19th 860,000 12 Hyg. 18° 

 " 22d 910,000 " " 27° 



The coatings were composed of an amalgam of tin and mer- 

 cury. 



Internal resistance of glass. — Very numerous measurements 

 were made on plates of ordinary glass, and on others colored 

 by oxide of cobalt, and even one plate of glass 6'2™°' in thick- 

 ness was employed. The current traversed all of these with a 

 certain facility, but the resistance of the cement used to attach 

 the coatings increased from week to week, and consequently it 

 is not worth while to give any figures at present. The internal 

 and external resistances of two excellent Leyden jars of recent 

 German make were also measured; in both cases the external 

 resistance was very considerably greater than the internal ; 

 hence, in use, the main leakage would be through the sub- 



