602 Critical Velocities for Electrons in Helium. 



were able to make a bend show up at this latter point under 

 conditions when it could not be due to a second radiating 

 collision at twice the resonance velocity. A series of obser- 

 vations in which this result was obtained are plotted in the 

 curve of fig. 6. In taking these observations a potential 















Fig. 



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 S 

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10 20 30 40 50 



Electron velocity (volts) 



60 



70 



difference of 7 volts was maintained between the gauzes D 

 and G (which in most of the earlier experiments had been 

 at the same potential) in such a direction as to drive positive 

 ions towards the collecting electrode A, and the potential 

 difference V 3 was in the direction for radiation, as well as 

 ionization, to give a positive charge to the electrometer. 

 The gas-pressure (0*170 mm.) was too low for it to be likely 

 that the bend at 41 volts is due to multiple collisions, and it 

 may therefore be taken to indicate a new critical velocity at 

 this point. An upward bend at about 41 volts was obtained 

 in several curves taken under conditions similar to those 

 stated. In some experiments at still lower pressures (about 

 01 mm.) taken with the final field V 5 reversed, a bend in 

 the opposite direction was obtained, showing that radiation 

 was produced at this point, and that at this low pressure 

 its photoelectric effect on the cylinder was greater than the 

 ionization it produced in the gas. 



The curves in figs. 5 and 6 may thus be interpreted as 

 supporting the view that at 41 volts and at 55 volts electron 

 collisions with positively charged helium atoms occur, which 

 result in an increased positive current being measured by 



