760 Prof. F. Horton and Miss A. C. Davies on 



electron stream, an effect which has been fully investigated 

 in an earlier paper *. It will be seen that the measured 

 current increases slightly between 20'6 volts and 21*2 volts, 

 which is the effect of the 20*4 volts radiation. 



These experiments therefore show that in pure helium 

 radiation is produced at 20'4 volts, and that a second type of 

 radiation is produced at 21 - 2 volts. In addition ionization 

 is produced at 21'2 volts, which, in the case of the higher 

 gas pressures now used, is considerably in excess of the 

 photoelectric current due to radiation. Evidence is given 

 later in the paper that this ionization is a cumulative effect, 

 and that it arises from the action of the 21*2 volts radiation 

 upon helium atoms which are in an abnormal condition 

 owing to the absorption of 20*4 volts energy. 



The absorption and re-emission of the two types of 

 Radiation. 



Compton considered the ionization which he detected be- 

 tween 20 -2 volts and 25*5 volts to be due to the cumulative 

 effects of an absorption of radiation and a direct electron 

 impact. Actual multiple electron collisions with helium 

 atoms would occur only very rarely, except under very 

 special conditions ; but, if a normal helium atom is capable of 

 absorbing and subsequently re-emitting the radiation from 

 an atom which has been stimulated to the radiating condition 

 by actual collision with an electron having the requisite 

 minimum energy, this radiation may be handed on from 

 atom to atom, and the possibility of collisions occurring 

 between abnormal atoms and electrons should be increased 

 thereby. Compton considers that his experiments show that, 

 even at low pressures and with moderate intensities of 

 electron emission, a sufficient number of such collisions occur 

 to give an appreciable ionization current. 



In putting this explanation of the origin of the ionization 

 to the test, the truth of the assumption with regard to the 

 absorption and subsequent re-emission of the radiation result- 

 ing from 20*4 volt electron collisions was investigated by 

 direct experiment. Electrons from the side filament, H, were 

 accelerated towards the gauze K by means of a potential 

 difference which could be varied by tenths of a volt. 

 Between the gauzes K and L the electrons encountered an 

 opposing potential difference, V 6 , greater by 4 volts than 

 that which accelerated them, and so they were prevented 

 from reaching the gauze L. Between L and M a potential 

 difference V 7 was applied so as to prevent positive ions 

 from reaching M. Thus the original electrons and any 



* Proc. Roy. Soc. A xct. p. 408 (1919). 



