574 On the Ionization Potential of Helium. 



proportion is about what would be expected from the dimen- 

 sions of the apparatus and from the pressure of the helium 

 used. It seems doubtful whether electron collisions with 

 helium under the ionizing potential are perfectly elastic, but 

 it is difficult to draw definite conclusions about this question 

 from the present experiments. 



Fig. 4. 



% 140 

 a 

 -fVl30 



..120 



Ho 



gllO 



5 



"fc 100 



* eo 



70 



60 

 50 













i 



X 















4 

 l 













































'« 





























( 



\s" 















1 



.-- 













^^ 































io ^ 



3 



4;0 5.0 6;0 7 



1 



■ 8 90 

 4— Volts 



o 



cu 



Q> 80 

 |^70 



] 60 

 50 



























^-"" 



A 











^*3>- 



^— A 













' ■ o ' 



— 







, 







10 



20 



40 50. 60 < — l/o/; 



The curves of fig. 4 also indicate strongly that the true 

 ionization potential of helium is not more than 20 volts, 

 rather than 20*5 or 21 volts. This follows from the fact 

 that the jump in ionization is seen already to have taken 

 place at 40, 60, and 80 volts, at which potentials the 

 correction to allow for initial velocities is relatively 

 unimportant. 



In conclusion, it is to be observed that the results obtained 

 in these experiments cannot be ascribed either to the ioni- 

 zation of impurities — for there were no impurities present in 

 the critical experiments — or to the liberation of electrons 

 from the metal parts of the apparatus, for this would have 

 registered a positive charge on the electrometer. It would 



