Occurrence of Ionization by Cumulative Effects. 1021 



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11-4 



In considering the origin of any ionization obtained below 

 the normal ionizino- voltaoe of helium under the conditions 

 of his experiments, Prof. Compton dismisses the possibility 

 or! its being due to electron impacts against helium atoms in 

 such rapid succession that the energies of the impacts are 

 additive in their effect, for reasons given by him in another 

 paper *. He explains the ionization by supposing that part oE 

 the necessary energy is acquired from the radiation produced 

 at the minimum radiation voltage, which is assumed to be 

 absorbed and re-emitted by other atoms and thus passed on 

 from atom to atom. That this assumption is justified has been 

 shown by recent experiments of the writers f . It is, however,, 

 upon this very property of the radiation (which is so essential 

 to Prof. Oompton's interpretation of his results) that the writers 

 based their main criticism of: his deduction that ionization o£ 

 helium was occurring at all the pressures given in the table^ 

 and that the ratio of the ionization current to the radiation 

 current at each pressure in his experiments is that stated in 

 the third column. 



Prof. Oompton's criterion for the presence of ionization 

 was the deviation of the experimentally determined value of 

 the ratio R from the value which he took to be the standard 

 one for a pure radiation effect, viz. P=l/c = 2. Any error 

 in the value taken as the standard seriously affects the in- 

 terpretation of the experimental results, and it was pointed 

 out in our criticism of these experiments that the occurrence 

 of: the process of the handing on of the helium radiation 

 from atom to atom of necessity means that the area of the 

 collecting electrode acted upon by the radiation in any given 

 case is not simply the area of the covered part of the 

 particular plane end of this electrode which is facing the 

 filament, but may include also the area of: the curved surface 

 of: the cylinder and possibly even the area of the plane end 



* K. T. Compton, Phys. Rev. vol. xv. p. 476 (1920). 



t F. Horton and A. C. Davies, Phil. Mag-, vol. xlii. p. 746 (1921 j. 



