'224: Prof. Townsend on the Conductivity "produced in 



is, according to this theory, 4*3 x 10~ 5 centimetre. In going 

 through this distance, the ions only acquire a velocity, cor- 

 responding to a fall of potential of 2186 x 4-310~ 5 ( = 'i) volt, 

 which is about 1*5 times the velocity of agitation (Section 5, 

 equation (5)). When the velocity of an ion before impact is 

 nearly the same as the velocity of agitation, the effect of the 

 collision on the ion may reverse the direction of motion 

 without diminishing the velocity. Under the present cir- 

 cumstances it is to be expected that some of the ions move 

 back towards the plate after colliding with molecules of the 

 gas, and lose their charge by coming into contact with the 

 surface at which they were generated. 



In order to ensure that nearly all the ions generated by 

 the light should get into the gas, it would be necessary to 

 have the electric force above a certain value. It would 

 appear from the experiments that a force of 2186 volts per 

 centimetre would be sufficient for this purpose when the 

 pressure is 69 millims. The fall of potential in a length equal 

 to the mean free path is about a volt in this case, and the 

 velocity acquired is five times the velocity of agitation. 

 Much smaller forces would be necessary for lower pressures 

 in order that the fall of potential in a length equal to the 

 mean free path should be of the order of- one volt; thus we have 



X X 



= 1 or — = 21. It will be seen from the curve fig. 6 



pxj3 p < x 



that the value of a x corresponding to the value of —=21 is 



very small. P 



Hence if the force X is constant and the pressure gradu- 



"V" 



ally lowered the conductivity increases slowly until — is 



about 20 ; at this point practically all the ions generated by 

 the light traverse the air-space between the plates. Further 

 diminution in p produces an increase in conductivity owing 

 to the formation of new ions by collisions, the increase being- 

 very great for the larger distances between the plates. When 



is about 372, the value of a is a maximum and p is the 



V X 



critical pressure ; for values of p less than ~—- the number 



of ions formed by collisions begins to diminish ; and finally, 

 when the pressure is very small, the conductivity approaches 



"V" 



the value corresponding to — = 20. 



12. We may here compare some of the conclusions at which 

 Ave have arrived with the results obtained from the coeffi- 

 cients of diffusion of ions. 



