Critical Velocities for Electrons in Helium. 593 



which indicate critical points, may be well marked. We 

 liope to make further experiments using an apparatus in 

 which the electrons in the primary stream are sorted out by 

 means of a magnetic field, so that all those which enter the 

 ionization chamber have the same velocity. The results 

 •contained in the present paper, which can only be considered 

 as approximate, are given for comparison with the recently 

 published work of Franck and Knipping *. These authors 

 have obtained values in agreement with those which we 

 found for the minimum electron velocities for the production 

 of radiation and of ionization in helium, and, in addition, 

 have produced evidence that a third type of collision, 

 resulting in an increase of ionization, occurs when the 

 A r elocity of the impacting electron is raised to 79*5 volts. 

 In determining this latter point the difficulties we have 

 mentioned, of obtaining really marked bends in the current 

 •curves with high electron velocities, were also experienced. 



The arrangements used in the present experiments have 

 been described in detail in earlier papers, but the method 

 ■can be understood by reference to the diagrammatic repre- 

 sentat ; on in fig. 1. Electrons from the glowing filament F 



Fig. 1. 



were accelerated towards the surrounding electrode E by a 

 difference of potential, V 1? which was constant during any 

 particular series of observations. Those electrons which 

 passed through the small hole in the centre of the top of E, 

 were further accelerated by the potential difference V 2 , 

 which was varied during the experiments. The potential 



J. Franck and P. Knipping, Phys. Zeits. xx. p. 481 (1919). 



