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LXXXII. The Production of Radiation and Ionization by 

 Electron Bombardment in Pure and in Impure Helium. 

 By Feank Horton and Ann Catherine Davies *. 



Introduction. 



IN the Proceedings of the Royal Society for 1919 

 (A, vol. xcv. p. 408) the authors have given an account 

 of an investigation of the effects of electron collisions with 

 helium atoms. It is there shown that in pure helium an 

 electron having energy corresponding to 20*4 volts is able to 

 produce radiation from a helium atom on collision with it, 

 and that when the energy of the impacting electron has 

 been increased to 25*6 volts it is able to ionize the gas. 

 These results have been confirmed by the work of Franck 

 and Knipping f , of K. T. Compton J, and of Goucher §. 

 Franck and Knipping gave the values 2O5 + 0'25 volts for 

 the minimum energy required to produce radiation, and 

 25*4 + 0'25 volts for that required to produce ionization; in 

 addition, they interpreted their results as indicating the 

 existence of a second radiation point 08 volt higher than 

 the one already mentioned, in agreement with the difference 

 to be expected from the existence of the two serial systems 

 in the helium spectrum. 



In a recent paper \) Horton and Bailey have shown that 

 the presence of a slight trace of impurity in helium enables 

 ionization to be detected at about 21 volts, and they found 

 photographically that the ionization they detected below the 

 ionization potential difference for normal helium was not 

 ionization of the helium itself, but ionization of the impurity 

 present. That such a result might be expected had been 

 pointed out by Bohr, for the ionization of all likely im- 

 purities occurs below 20' 5 volts, so that the frequency of the 

 helium radiation is sufficiently high to ionize any impurities 

 likely to be present in the gas. 



Both Compton and Goucher, however, in the papers referred 

 to, record the detection of ionization as well as of radiation at 

 about 20'4 volts. Goucher attributes this ionization to the 

 presence of impurity, probably hydrogen, but Compton 

 accounts for it as a cumulative effect resulting from electron 

 impacts on helium atoms which have already absorbed 



* Communicated hy the Authors. 



t J. Franck and P. Knipping, Phys. Zeits* xx. p. 481 (1919). 



t K. T. Compton. Phil. Mag-, xl. p. 553 (1920). 



§ F. S. Goucher, Proc. Phvs. Soc. xxxiii. p. 13 (J 920). 



|| F. Horton and D. Bailey, Phil. Mag. xl. p. 440 (1920). 



