the Ions in Gases at Loiv Pressures 



55?, 



if d represents the distance between the plates, X the value 

 of the electric field when the rate of leak first begins to be 

 affected by the magnetic force H. Assuming that the field 

 is uniform, 



X = Y/d, 



where V is the potential-difference between the plates ; and 

 equation (1) becomes 



,_'2Vm 

 d '~ eW ' 



The negative ions travelling between the plates will disturb 

 to some extent the uniformity of the field between the plates ; 

 but if the intensity of the ultra-violet light is not too great, 

 so that the rate of leak and the number of ions between the 

 plates is not large, this want of uniformity will not be 

 important. A calculation of the amount of variation due to 

 this cause showed that its effect was not large enough to 

 make it worth while correcting the observations for this 

 effect, as the variation in the intensity of the ultra-violet 

 light was sufficient to make the errors of experiments much 

 larger than the correction. 



The following is a specimen of the observations : — 

 Distance between the plates "29 centim. 



Strength of magnetic field 164. Pressure 1/100 millim. 



Potential-difference 

 between Plates, in 



Deflexion of Electrometer in 30 sees. 







TOltS. 



Magnet off. 



Magnet on. 



240 



180 



190 



120 



160 



165 



80 



160 



140 



40 



130 



75 



These observations showed that the critical value of the 

 potential-difference was about 80 volts. A series of observa- 

 tions were then made with potential-differences increasing 

 from 80 volts by 2 volts at a time, and it was found that 90 

 volts was the largest potential-difference at which any effect 

 due to the maguet could be detected. The results of a number 

 of experiments are given in the following table : — 



