Critical Velocities for Electrons in Helium. 601 



effects. The electron velocity at which the bend in the 

 curve is to be expected (40*6 volts) is almost identical with 

 the point at which, under suitable pressure conditions, an 

 increase of radiation should be produced by the second 

 collisions of electrons with normal helium atoms, but the 

 two effects can be distinguished, since the conditions which 

 favour the occurrence of ionization, and which therefore 

 favour the production of the 40*62 volt radiation, are those 

 under which increases of radiation at multiples of the 

 resonance velocity would not be likely to occur to any 

 appreciable extent. 



In considering the possibility of detecting the production 

 of a new type of radiation or of fresh ionization from the 

 collisions of electrons with helium atoms which have already 

 lost one electron, the effects of recombination must be taken 

 into account. It is conceivable, for instance, that recom- 

 bination takes place at all encounters between positively 

 charged helium atoms and electrons with velocities below 

 that necessary for the prpduction of radiation from the 

 charged atom — 40'62 volts on Bohr's theory. This being 

 the case, an increased positive current should be measured 

 by the electrometer at 40' 62 volts on account of the 

 reduction at this point in the amount of recombination 

 occurring. Whether in this case an increased positive 

 current would also be obtained when the electron velocity 

 is raised to that necessary to remove the second electron 

 from the ionized atom is uncertain, and would depend 

 upon whether the ionization then produced were more 

 than that which the 40" 62 volt radiation was producing from 

 the normal helium atoms. 



Again, it is possible that an electron which gives rise to 

 40*62 volt radiation itself recombines with the positively 

 charged helium atom with which it collides. In this case 

 an increased positive current should be obtained when the 

 critical velocity for ionization of the positively charged helium 

 atom is reached, since the encounters would then result in 

 the production of doubly charged helium atoms instead of 

 neutral ones. As the precise conditions under which recom- 

 bination of a colliding electron and a positively charged 

 helium atom on which it impinges occurs are unknown, it is 

 impossible to predict how the existence of critical velocities 

 at 40*62 volts and 54*16 volts for this atom should be 

 indicated. The curve in fig. 5 shows a bend which we have 

 attributed to the further ionization of the positively charged 

 helium atom, but no bend at all in the neighbourhood of 

 40 volts. By altering the experimental arrangements we 



Phil. Mag. S. 6. Vol 39. Nf . 233. May 1920. 2 R 



