222 Prof. Townsend on the Conductivity produced in 



When E is expressed in volts and I in centimetres, I find 



E 

 from the above numbers that the mean value of 7- is 372. 



So that if X is the force in volts per centimetre, the value of 



p for which the conductivity is a maximum is p= — -. 



In Section 7 we found that the value of p for which a is a 

 maximum when X is constant is given by the equation 



p— ^™ so that the empirical law discovered by Stoletow is 



completely explained by the theory of collisions. 



The other results obtained by Stoletow can also be ex- 

 plained, but the numerical coincidence is not very accurate 

 in some cases. The discrepancies occur when the con- 

 ductivity between the plates is very small, and it is possible 

 that the experimental results are not so accurate in these 

 cases, but on the whole there is a very good agreement 

 between theory and experiment. 



Let us consider how the current depends on the distance 

 between the plates when the conductivity is produced bv 

 ultra-violet light. The following table gives some of the 

 results obtained by Stoletow when the difference of potential 

 between the plates was proportional to their distance apart I. 

 For the largest distance (1*08 millim.) the potential was 165 x 

 1-43 volts. 



p- 



1= 108 



•655 



•393 



•262 



750 



i = 7-62 



741 



7-39 



7-33 



69 



1837 



17-99 



17-82 



16-76 



7-7 



491 



112-7 



48 ; 2 



32-7 



For the larger pressures an increase in the distance between 

 the plates has little effect on the current; but for the pressure 

 7*7 millims. the current becomes very large when the distance 

 between the plates is increased. If we suppose as before 

 that the ions created by the action of the light start from the 

 plates, then the current should satisfy the equation (1), 

 Section 4, 



where I denotes the distance between the plates, and « is a 

 constant. 



