Discharge of Positive Electrification by Hot Metals. 99 



capacity and shifting of the zero, and can be arranged for 

 the proposed experiment (as I shall explain) so that no con- 

 necting wires need be exposed to the air. It can be insu- 

 lated by means of fused quartz in the form of rod and tube. 

 In this way there is practically no limit to the time during 

 which the leak may be allowed to proceed. It would be 

 quite practicable, if desired, to measure a leakage from a 

 charged wire at 100 volts not exceeding 10~ 18 ampere. 



Fig. 1 (p. 100) represents the apparatus, a is the high-poten- 

 tial wire carried by the brass cap b, which in its turn is 

 cemented into the quartz tube d. d forms a prolongation of 

 the neck of the glass vessel c, which incloses the high-poten- 

 tial wire a and the thermometer k. The further end of a is 

 supported by a quartz rod g, carrying a cup on the end in 

 which the wire rests. 



The glass vessel c is silvered inside, and connected to earth 

 through a platinum wire /. The brass cap b presses against 

 the brass strip p, which carries the gold-leaf o. This strip 

 and gold-leaf are inclosed in a glass vessel very thinly 

 silvered inside to make it conducting, and dried by phosphoric 

 anhydride contained in m. This vessel can be exhausted, as 

 shown in the figure. 



The arrangement of the electroscope is very similar to that 

 employed by Wilson (loc. cit.). The brass strip p is supported 

 by a quartz rod q, and this quartz by a brass rod s passing 

 out through the indiarubber cork t. p and s can be brought 

 into temporary metallic contact by means of the iron wire r 9 

 movable by an external magnet. To charge the wire a and 

 the electroscope it is merely necessary to give a suitable 

 charge to s (this charge being adjusted to a convenient value 

 by means of an auxiliary electroscope), and then to make 

 contact with p by means of the iron wire r. When a 

 suitable charge has been given the magnet is removed and 

 the contact thus broken, s is then touched to discharge it. 

 When c and the electroscope were exhausted insulation was 

 so good that the potential of the system did not sink more 

 than a fraction of a volt in 24 hours when initially charged to 

 about 100 volts. 



In order to raise the temperature of the w 7 ire a, the vessel 

 c was inclosed in a metal oven heated by a gas-burner. In 

 this oven was also placed an elongated glass bulb containing 

 air. This worked a temperature-regulator in the ordinary 

 way, cutting off the gas by its expansion when the tempe- 

 rature rose too high. 



The neck / of the vessel was long enough to prevent the 

 electroscope getting seriously heated by the radiation from 



112 



