﻿to maintain a Current in a Gas at Low Pressures. 213 



parallel, the gas being able to flow freely from one to the 

 other ; but no difference in the results obtained from the 

 different electrodes was eventually noticed, although at first 

 there was a decided effect due to the material. (See Part II.) 



The walls G G were formed by a ring of plate-glass accu- 

 rately ground and polished, approximately 1 cm. thick, the 

 hole which formed the spark-chamber being 3 cms. in 

 diameter. The plates were stuck to the glass with ordinary 

 commercial elastic glue, as was also the fine brass tubing a a 

 into the glass tubes b b, the joints being carefully tested to 

 see whether they were air-tight. These were the only joints 

 made in this way, all the rest of the apparatus being glass, 

 and the whole being frequently tested for leaks. The current 

 was measured by an ordinary d'Arsonval galvanometer, of 

 resistance 5'5 ohms, and giving 1 mm. deflexion on a scale 

 1 metre distant for 10 ~ 6 ampere. The potential was read 

 off on a multicellular electrostatic voltmeter, graduated up 

 to 600 volts and connected directly to the electrodes. A 

 battery of small storage-cells furnished the current, a large 

 liquid resistance being always in the circuit. It was at first 

 decided to use air in the spark-gap, but it was found that one 

 could not be certain of steady conditions with this gas, as the 

 spark would often flicker and suddenly become unsteady, and 

 would not always fill the whole space. This was most pro- 

 bably due to an effect on the electrodes, so it was decided to 

 use hydrogen throughout the investigation as it is so easily 

 obtained in a pure state. The gas was prepared electrolytically 

 from a solution of very pure Ba(OH) 2 in a generator specially 

 constructed to prevent any possibility of the hydrogen being 

 contaminated by impurities. The gas, after being evolved, was 

 carefully dried by allowing it to stand at least twenty-four 

 hours in close contact with dry phosphoric pentoxide before 

 being admitted to the spark-chamber. In connexion with this, 

 too, there was a drying-tube; so it would seem that sufficient 

 precautions were taken to ensure the gas being dry and 

 pure. 



For about four months, while the final readings were being 

 taken, no air sufficient to be detected bv the McLeod gauge 

 came in contact with the plates at any time. To make sure 

 of the continued purity of the gas between the plates, it was 

 entirely cleaned out every few days, the pumping being- 

 carried down to at least -^ mm., and fresh gas was admitted. 

 During the running of the current there was sometimes a 

 slight absorption of gas by the electrodes, this and similar 

 effects being noted below ; but the method of taking the 

 readings prevented any serious inaccuracy due to this cause. 



