of Radiation by Electron Impacts in Helium. 5. r )5 



for by the photoelectric effect on the electrodes produced 

 by the resonance radiation. This lias been con finned inde- 

 pendently by Horton and Miss Davies* and by Franck and 

 Knippingt- These investigators used the well-known method 

 of Davies and GoucherJ for distinguishing between the 

 effects of ionization and of resonance radiation, while Franck 

 and Knipping also used the total and partial current method 

 of Tate and Foote§. Both sets of investigators found the 

 true ionization potential to be at about 25'5 volts, the actual 

 values published being 25*7 and 25*3 volts respectively. 

 Horton and Miss Davies further concluded that the effect at 

 20'4 volts is one of pure radiation — i. e., there is no accom- 

 panying ionization. 



On the other hand, Franck and Hertz [| have pointed 

 out that the successive maxima and minima observed by 

 them at multiples of 20*4 volts cannot be adequately ex- 

 plained by a photoelectric effect of the radiation on the 

 electrodes, but prove an actual production of ions in the 

 gas in the region where the collisions become inelastic. 

 Furthermore, Ren tschler 11 has obtained results which cannot 

 reasonably be accounted for by a photoelectric effect setting 

 in when the accelerating potential exceeds 20*4 volts, but 

 indicate an actual ionization at this voltage. Rentschler 

 attributed this ionization to ionization from the surface 

 of his gauze electrode, rather than to a true ionization 

 of the gas, and concluded that helium exhibits no reso- 

 nance at 20 volts, but only ionization at 25'5 volts. Finally, 

 the experiments made by Benade and Compton **, and espe- 

 cially byBazzonift, in which the electron current between 

 two electrodes was found to increase abruptly as the accele- 

 rating potential reached multiples of 20 volts, can only 

 be accounted for on the assumption of actual ionization of 

 the gas at 20 volts, since the effects observed were entirely 

 too large to be accounted for as a photoelectric effect by that 

 portion of the resonance radiation which could have fallen 

 on the emitting cathode. 



The present research was undertaken to investigate the 

 resonance and ionization potentials of helium by a method 



* Rov. Soc. Proc. A. xcv. V . 408 (1019). 

 t J'hys. Zeit. xx. p. -181 (1919), 



% Lor. at. 



§ Phys. Rev. vii. p. 696 (1916) ; x. p. 77 (1917). 



j| Piii/s. Zeit. xxx. p. 132 (1919). 



% Phys. Rev. xiv. p. 504 (1919). 

 ** Phys. Rev. xi. p. 184 (1918). 

 tt Loe. cit. 



2 2 



