414 Messrs. Stead and Grossling on Relative Ionization 



of the field due to the electrons at the time in transit across 

 the space. (Space-charge effect.) It' the electrons are pro- 

 jected with negligible velocity from an equipotential cathode, 

 it can be shown* that- the current will vary as the 3/2 power 

 of the applied voltage up to the point when the current 

 reaches a saturation value dependent on the temperature of 

 the cathode, as shown in fig. 1. The current-voltage cha- 

 racteristics of tubes in which the voltage is great enough for 

 the corrections for initial velocity and for variation in the 



Fiff. 1. 



potential of the starting point on the cathode surface to be 

 negligible, afford ample and accurate confirmation of this 

 theory, which has further been applied with uniform success 

 in the design of more complicated instruments, such as 

 high-voltage thermionic valves. 



When, however, the applied voltage is limited to low 

 values, as in the case of the investigation of ionization effects, 

 the effects of initial velocity and variation along the cathode 

 are no longer to be neglected. In these experiments the same 

 cathode heated to approximately the same temperature was 

 used throughout. The results therefore indicate values of the 

 ionization potential relative to any one of the gases observed 

 taken as standard. It is, however, necessary to assume 

 that replacement of one gas by another does not give rise to 

 serious changes in the contact potential between the emitting 

 and collecting electrodes. 



The tube used was a three-electrode valve of the well- 

 known form, the cold electrodes being connected together 

 for the purposes of the experiments. Next the tungsten 



* Physical Review, vol. ii. p. 455 (1913). 



