206 Prof. Townsend on the Conductivity produced in 



would not cease when the rays are stopped. We must 

 therefore attribute the increase in conductivity to ions of 

 one kind ; and it can be clearly shown from experiments 

 with one electrode inside the other, that it is the negative ions 

 which produce the large increases in conductivity. 



Fig. 6. — Pressure -171 mm. 



220 



2O0 



ISO 



160 



140 



(20 



30 



60 



40 



20 



r 



80 160 240 320 400 



Volts per centimetre. 



640 720 



4. The first step towards an explanation of these results is 

 to find from the curves the number, a, of ions of one kind 

 that a single negative ion will generate in moving under an 

 electric force through a centimetre of the gas. The number 

 of positive ions which are generated is equal to the number of 

 negative ions, so that in what follows we need consider only 

 the negative ions ; it being understood that when a negative 

 ion is produced, a positive ion is also produced. 



Let a force X be applied to N negative ions in a gas at 

 pressure p and temperature t. Let N be the total number 

 of negative ions after the N ft ions have travelled a distance x m 



