410 COMPARISON OF RESISTANCES. 



The experiments are very regular with phosphorus, but the 

 numbers obtained for sulphur vary considerably, as do its other 

 physical properties, with the previous conditions through which it 

 has passed. The specific resistance of phosphorus at 50 is i^.io 6 

 ohms; from 25 to 100 it diminishes in the ratio of 6 "6 to i. The 

 specific resistance of sulphur at 120 is about io 10 ohms. 



Dry glass may be regarded as a perfect insulator at ordinary 

 temperatures. Cavendish, however, had observed that at 300 it 

 allows electricity to pass ; the decrease of resistance is very rapid, 

 and a difference of 6 to 9 is sufficient to double its conducting 

 power. The resistance of various glasses may be represented, 

 according to Foussereau, by an expression of the same form as 

 that for sulphur and phosphorus. 



It results from these experiments that plate glass has a much 

 greater resistance than ordinary glass, and this, again, than Bohemian 

 glass ; for the specific resistances of these three bodies at 50 are 

 respectively 34io.io 12 , 2-4. io 12 , and o'3.io 12 ohms. It is to be 

 presumed that the conductivity of heated glass is of the same order 

 as that of electrolytes, and that the passage of a current is accom- 

 panied by chemical phenomena. 



Experiments with gutta-percha are more difficult, owing to phe- 

 nomena of electrical absorption, but the conductivity manifestly 

 increases with the temperature. According to Bright and Clark, 

 the resistance between the temperatures o and 24 may be repre- 

 sented by the formula 



f=r 0-8878'. 



1006. GASES AND VAPOURS. It is known that gases and vapours, 

 which are absolute insulators at low temperatures and under the 

 atmospheric pressure, become conductors at high temperatures. 

 Volta has observed that the flame of a candle or of a sulphur 

 match acts like an infinitely fine point, and allows electricity to 

 escape, at any rate for high potentials. Sir W. Thomson, utilising 

 this property of heated gases, employs a slowly- burning strip of 

 paper impregnated with nitrate of lead as an equaliser of potential, 

 which is very useful in studying atmospheric electricity. 



In like manner, the electric current in an arc light passes by the 

 layer of hot gases and vapours which separates the two carbons. 

 This is also the case with electrical sparks in air, and discharges in 

 tubes of rarefied gas. The resistance of the gas diminishes with the 

 pressure to a certain minimum, corresponding to a pressure of a few 

 millimetres of mercury, which varies in different gases ; it then 



