568 Ionization by Electron Impacts in Helium. 



as the electronic mean free path is greater than the distance 

 between electrodes, or even if it is considerably smaller. It 

 is only when an electron is likely to collide while its velocity 

 increases by an amount equivalent to the difference between 

 19*5 and 20*2 volts that the influence o£ the neon should 

 increase in proportion to that of the helium as the pressure 

 increases. In the actual case, however, the ratio i/r changed 

 more than tenfold with pressure — at pressures where the 

 difference in probability of collision at 19*5 volts and at 

 20*2 volts is absolutely negligible. Furthermore, Richardson 

 and Bazzoni have shown that the amount of ionization at 

 multiples ot 20 volts is about what would be expected if every 

 electron, colliding against a helium atom with energy greater 

 than 20 volts, should directly or indirectly liberate an addi- 

 tional electron. This amount of ionization could not, therefore, 

 be attributed to collisions with atoms of an impurity present 

 in small quantity. The first two objections above apply 

 equally well to disprove a direct photoelectric action of the 

 helium-resonance radiation on neon atoms in sufficient 

 amount to explain the results. Richardson's and Bazzoni's 

 work, together with the observed passage of resonance 

 radiation through the gas to produce a photographic effect 

 on the electrodes, also proves the inadequacy of an expla- 

 nation based on a photoelectric emission from neon. 



Note added to proof. — Moi e recent experiments have been 

 made in helium so pure as to show no trace of impurities under 

 any conditions of exciting the spectrum. These experiments 

 differ in no way from those described above. 



Summamj. 



1. An experimental method is described for distinguishing 

 between ionization and radiation and of estimating the pro- 

 portion of either when both are present. 



2. Resonance radiation sets in at 20 2 volts and ionization 

 at 25-5 volts. 



3. Ionization is observed between 20*2 and 25*5 volts, in 

 proportions increasing with tne gas-pressure and with the 

 bombarding current density. Evidence is presented to show 

 that this ionization is a secondary effect, due to impacts 

 against electrons which contain absorbed radiant energy of 

 the resonance radiation from neighbouring atoms. This 

 method of ionization appears to be very important at high 

 gas-pressures. 



Princeton, N. J., U.S.A. 

 April 1, 1920. 



