L 440 J 



XLIX. The Effect of a Trace of Impurity on the Measurement 

 of the Ionization Velocity for Electrons in Helium. By 

 Frank Horton, Sc.D., Professor of Physics in the Uni- 

 . versity of London, and Doris Bailey, ALSc, Assistant 

 Lecturer in Physics in the Royal Holloway College, Engle- 

 field Green*. 



IT has been found by Horton and Da vies f that radiation 

 is produced when electrons having a velocity of 20*4 volts 

 collide with helium atoms, and that this effect is not accom- 

 panied by any ionization of the gas. Ionization of helium 

 was found to occur when the electron velocity was raised to 

 25'6 volts. An account of the experiments from which 

 these results were obtained was given to Section A of 

 the British Association at the meeting in Bournemouth 

 in 1919, and in the discussion which followed, the view- 

 was expressed by some of the speakers, that ionization of 

 helium occurs at the lower critical velocity mentioned above. 

 Dr. Goucher reported that experiments made by him with 

 an improved form of his original apparatus for distin- 

 guishing between the photoelectric effect of radiation on the 

 electrodes and the ionization of the gas by electron impacts, 

 had resulted in the detection of some ionization as well as 

 radiation at about 20 volts, and copious ionization at about 

 25*5 volts. He stated that the helium used by him was 

 possibly not as pure as that used in the experiments of 

 Horton and Davies. The evidence of other speakers was 

 mainly to the effect that when helium was used in a therm- 

 ionic valve the current-E.M.F. curves showed a rise at about 

 20 volts which could only be due to the production of ionization 

 at that point. 



The ionizing velocity for electrons in helium was first 

 investigated by Franck and Hertz, who gave 20*5 volts as 

 the critical value at which ionization occurs J. It was 

 pointed out by Bohr that the method used by these experi- 

 menters was not capable of distinguishing the effects of a 

 production of radiation in the gas from those of ionization of 

 the gas by electron collisions. From theoretical considerations 

 Bohr had deduced the value 29 volts as the minimum ioni- 

 zation velocity of helium, and he suggested that the effect 

 detected at 20'5 volts was not a genuine ionization of the 

 helium, but a secondary effect due to the production of 

 radiation in the gas. He pointed out that such a radiation 



* Communicated by the Authors. 

 + Proc. Roy. Soc. A. vol. xcv. p. 408 (1919). 



J J. Franck and G. Hertz, Deutsch. Fhys. Ges. Verh. vol. xv. p. 34 

 (1913). 



