Critical Velocities for Electrons in Helium. 599 



value agrees with the value deduced from Bohr's theory. 

 Iti the course of our investigation we obtained direct experi- 

 mental evidence of the existence of a critical velocity at 

 about this point. It has been mentioned that in most o£ 

 our experiments there was no difference of potential 

 between the gauzes D and 0, in. the space between which 

 most of the ionization occurred. In view of this fact, 

 when the ionization velocity has been exceeded, positive ions 

 will be distributed throughout the path of the primary 

 electrons, and it is possible that at a suitable pressure and 

 with a sufficiently intense electron stream, collisions take 

 place between ionized helium atoms and electrons with 

 considerable velocities. If a sufficient number of such 

 collisions occurred, the current-potential difference curve 

 would be expected to show a rise when further ionization 

 resulted from these encounters. An example of a curve in 



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ft 









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r 















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50. 



60 



30 40 40 



Electron vp/octy (volts) 



which an increase of current was obtained, which could not 

 be attributed to the production of radiation or ionization 

 by the successive collisions of an electron with different 

 helium atoms, is given in fig. 5. This curve was obtained 



70 



