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CVI. The Occurrence of Ionization by Cumulative Effects. 

 By Fkank Hokton and Ann Catherine Davies *. 



IN the Philosophical Magazine for March 1922, Professor 

 K. T. Compton replies to criticisms we made | of the 

 experiments by which he claims to have demonstrated the 

 ionization of helium below its normal ionizing voltage, as 

 the result of electron impacts against atoms which have 

 absorbed resonance radiation from neighbouring atoms J. 

 In this reply Prof. Compton produces evidence which he 

 states " proves these criticisms to be unfounded/'' 



The experiments which we criticized as giving inconclusive 

 results were made with an apparatus in which the collecting 

 electrode was in the form of a hollow cylindrical box having 

 one plane end of platinum gauze and the other plane end of 

 platinum foil, arranged so that each end in turn could be 

 faced towards the glowing filament. The ratio R of the 



currents measured in the two cases is -. , where i is the 



i + cr 



part of the measured current which is due to ionization 

 (which is the same whichever end of the box faces the fila- 

 ment), and r and cr are the parts of the measured current 

 which are due to the photoelectric effects of radiation when 

 the foil 'end and the gauze end, respectively, face the fila- 

 ment. Hence, if the constant c is known, the ratio of i/r can 

 be calculated from the ratio R of the two measured currents. 

 For pure radiation the value of R is 1/c, and for pure ioniza- 

 tion it is unity. Prof. Compton assumed that the value of c 

 was given by the ratio of the area covered by the wires of 

 the gauze end to the total area of the plane end, and checked 

 his calculation of this by comparing the photoelectric currents 

 measured when using radiation from a quartz-mercury arc 

 outside the apparatus. The value used by him in estimating 

 i/r was 0*5. The following table, which is taken from 

 Prof. Compton's paper, gives the values of R and i/r 

 obtained for electron energies between 20 and 25 volts, at 

 different pressures of helium : — 



* Communicated by the Authors. 



t F. Horton and A'. C. Davies, Phil. Mag. vol. xlii. p. 746 (1921). 



% K. T. Compton, Phil. Mag. vol. xl. p. 553 (1920). 



