of ike Ionization Velocity for Electrons in Helium. AA1 



single-grid valve having a lime-coated filament and containing 



helium contaminated with a trace of mercury vapour from the 

 sealing-wax used in coating the platinum strip. In taking 

 the observations recorded in this figure a constant difference 



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Electron velocity (volts) 



of potential of 8 volts was maintained between the anode 

 and the grid and the difference of potential between the grid 

 and the filament was gradually increased. The currents 



measured bv tin 



was gradually increased. 

 e galvanometer connected directly to the 

 filament are given (Curve I.), as well as those measured by 

 the anode galvamometer (Curve II.), the latter being also 

 plotred on a larger scale (Curve III.), so that the rise in the 

 current at about 21 volts may be seen. The curves show 

 that both the measured currents increased abruptly at an 

 electron velocity a few volts above that at which ionization 

 of the helium can occur. The current measured by the 

 filament galvanometer is mainly that carried by the stream 

 of electrons from the glowing lime, and the sudden rise in the 

 curve must be due to an increased emission, which no doubt 

 results from the neutralization of the space-charge near the 

 filament by the positive ions travelling to it. It was found that 

 the point at which the sudden large increase in the current 

 occurs depends very much upon the electric fields used 

 between the electrodes. It could be prevented by adjusting 

 the fields so as to prevent the positive ions formed in the 



