244 Mr M c Glelland, On the Conductivity of Gases 



conductivity due to recombination of the positive and negative 

 carriers, does slightly increase the current. 



If we diminish the velocity of the stream of gas past the 

 terminal we get the maximum current for a smaller E.M.F. because 

 now the gas takes longer to pass the terminal and so a given 

 E.M.F. can discharge all the carriers inside a greater radius. The 

 conductivity of the gas is entirely destroyed after it passes the 

 terminal B if this terminal has been kept at a potential sufficiently 

 different from that of the tube A. 



The conductivity therefore is such as can readily be explained 

 by the ionisation of the gas. 



(3) By charging the terminal B positively and negatively we 

 get not quite the same rate of leak in the two cases ; the terminal 

 leaks rather more quickly when charged negatively. This would 

 show that an excess of positive carriers comes off from the arc, 

 but this excess is small compared with the total amount of either 

 sign. 



It is shown afterwards that the negative carriers move with a 

 slightly greater velocity than the positive under an electric force, 

 and this would cause more of the negative carriers to be discharged 

 to the terminals of the arc, leaving an excess of positive in the 

 gas. The value that we have found for the velocity of the carriers 

 under an electric force is very small, and the difference in the 

 velocities of the positive and negative is also small, but these 

 determinations are made after the gas has left the arc. In the 

 arc itself the velocities of both negative and positive are probably 

 much greater. We know from Prof. Thomson's results 1 that 

 probably in all cases of ionisation the mass of the negative carrier 

 is initially very small compared with that of the positive, while 

 their charges are the same. In gas at atmospheric pressure the 

 carriers soon collect around themselves a very much greater mass, 

 so that the difference in the velocities of the positive and negative 

 under an electric force is greatly diminished. We have then in 

 the arc the negative carrier of much smaller mass than the positive 

 initially, but their masses soon becoming more nearly equal. In 

 the field of force near the arc terminals the negative carriers 

 produced by the passage of the discharge will therefore be dis- 

 charged to the terminals to a greater extent than the positive. 

 This explanation of the electrification of the gas produced by an 

 arc would not account for the negative electrification found in 

 hydrogen 2 , but Prof. Thomson points out that there are reasons 

 for ascribing the negative electrification in hydrogen to some 

 secondary action. 



1 See Phil. Mag., December, 1899. 



2 J. J. Thomson, Discharge of Electricity through Gases, p. 93. 



