Gases by the Motion of Negatively charged Ions. 221 



2. When the pressure and distance between the plates 



were constant, the current increased with the electric 

 force ; at very low pressures the current reached a 

 maximum, and remained constant for large forces *. 



3. When the electric force was constant, the conductivity 



attained a maximum value for a certain critical pressure, 



which was accurately proportional to the force and 



independent of the distance between the plates. 



If we suppose that a certain number of negative ions are 



supplied initially by the action of the light; on the metal 



surface, we can explain all these results bj the theory which 



we have been considering. Stoletow's experiments at low 



pressures show that the number of ions supplied initially 



cannot vary much with the pressure. For a given force the 



conductivity would therefore be a maximum for the same 



value of p as makes a sl maximum. 



We see therefore that the third result relative to the critical 



pressure agrees with the conclusion arrived at in Section 7 



relative to the maximum value of a, and moreover a remark- 



vr 



able numerical coincidence exists between the value of — for 



X I* 



which « is a maximum, and the value of — which gives the 



maximum current with ultra-violet light. 



The following table is given by Stoletow : — - 



E is the electromotive force between the two plates, the 

 unit being the electromotive force of a Clark cell. 



I is the distance between the plates in millimetres. 



p is the pressure for which the current is a maximum. 

 pi 



The values of WlO 4 for the different experiments are given 



in the fourth column. 



E(CL). 



/ (millim.). 



p (millim.). 



tfio*- 



E 



165 



'25 



25-3 



383 



165 



•47 



13-5 



384 



65 



•47 



5-3 



383 



100 



•83 



4-7 



389 



65 



•83 



30 



383 



60 



•83 



2-8 



386 



65 



1-91 



1-3 



382 



65 



3-71 



•67 



382 



40 



3-60 



•43 



387 



* This corresponds to the results obtained at -171 millim. pressure, aa 

 can be seen from the curves, fig-. 0. 



