of the Ionization Velocity for Electrons in Helium. 449 



and 2o'S volts, the time of exposure of the plate was from 

 4 to 5 hours. With an electron velocity of 50 volts a 

 bright luminosity was obtained and the exposure was only 

 about 1 hour. 



Velocity of the 



electron st renin. 



A 





21-4 volts. 23-7 volts. 



25-8 volts. 



50 volts. 





5048 (0-5) 



5048 (5) 





5016 (2) 



5016 (8) 





4922 (1) 



4922 (6) 





4713 (5) 



4713 (6) 





4472 (5) 



4472 (10) 





4438 (0-5) 



4438 (6) 





4388 (0-5) 



4388 (6) 



4859 very faint 4359 (2) 



4359 [1) 



4359 (5) 



434S (0-5) 





434S (1) 

 A337 (0 5) 

 4169 (1) 

 4144 (2) 

 4121 (3) 

 4026 (3) 

 3868 (0-5) 



The numbers in brackets indicate the relative intensities 

 of the lines on the photographic plate. The wave-lengths 

 printed in italics are due to mercury. 



The lines 4359 and 4348 are due to mercury, and these 

 were the only lines obtained so long as the maximum velocity 

 of the electron stream was less than 25 volts. The line 4337, 

 which was faintly visible on the plate when an electron 

 velocity of 50 volts was used, is also due to mercury, but 

 all the other lines given in the table are due to helium. On 

 the view, now generally accepted, that the many-lined 

 spectrum of a gas can be produced when ionization has 

 occurred, these results indicate that ionization of helium 

 does not occur below 25 volts, but that ionization of mercury 

 w 7 as taking place between the resonance and ionization velo- 

 cities of helium. From the first column of the table it will 

 be observed that the only line seen on the photographic plate 

 when the electron velocity was 21'4 volts was the mercury 

 line X4359, and as this was extremely faint, it is evident that 

 there was very little ionization of the mercury vapour by 

 direct impact, even although the electron velocity was well 

 above the ionization value for mercury (10*4 volts). The 

 greatly increased intensity of this line when the voltage 

 was raised to 23* 7 volts must therefore be due to the hugely 

 increased amount of helium radiation then produced. These 

 two spectra thus indicate that the amount of ionization of 

 mercury vapour by direct electron impact is extremely small 



