248 Mr M c Glelland, On the Conductivity of Gases 



current through the arc is increased we have more matter given 

 off from the terminals either by their disintegration or by the 

 escape of occluded gas, and thus the conditions are rendered more 

 favourable for the condensation of uncharged masses on the 

 carriers. 



(7) When the current through the arc is increased the density 

 of ionisation in the gas taken from it is of course increased, and 

 the electric force acting on the carriers in the field between the 

 terminal G and the tube A will differ from its assumed value by 

 an amount depending on the density of ionisation. This would 

 make the velocity deduced for the carrier to be less than its true 

 value, and the error would increase with the density of ionisation. 

 The amount of the error thus introduced was examined by first 

 determining the velocity with dense ionisation, and then cutting 

 down the ionisation by making the gas pass through a number of 

 layers of fine wire-gauze packed close together, before determin- 

 ing the velocity of the carrier. The result showed that the error 

 introduced in this way was small. 



(8) In all these determinations of the velocity of the carrier 

 the gas is practically at the temperature of that in the room at 

 the point where the velocity is measured ; the arc is small and 

 the hot gas from it is mixed with a large volume of cold gas so that 

 at a short distance from the arc the temperature has fallen almost 

 to that of the room. Some experiments were made to test the 

 effect on the velocity of the carrier produced by heating the gas. 

 The tube A, for this experiment, was not inserted directly in the 

 funnel F, but was connected with it by a glass tube bent twice at 

 right angles, so that the horizontal part of it could easily be 

 heated by a bunsen burner. The velocity of the carrier at the 

 terminal G could then be determined when the gas at that point 

 is at various temperatures. It was found that when the current 

 through the are was large, so that the velocity of the carrier was 

 small, the effect of heating the gas was to increase the velocity of 

 the carrier ; the increase of velocity produced by heating the gas 

 was much less when the current through the arc was small, and 

 therefore the velocity of the carrier greater. The smaller the 

 velocity of the carrier in the cold gas the greater was the increase 

 produced by heating it. This result would favour the view that 

 the decrease of velocity of the carrier is produced by more un- 

 charged material travelling with it. 



(9) Some experiments were made with brass terminals for 

 the arc in place of the platinum terminals. With the brass 

 terminals the velocity of the carrier was less than with platinum 

 terminals, and fell away even more rapidly when the current 

 through the arc was increased. The disintegration of the brass 



