44:4 Prof. F. Horton and Miss Doris Bailey on Measurement 



the gas, the currents to the anode were measured as the 

 accelerating potential difference was gradually increased, and 

 the currents were plotted against the corresponding values 

 of the electron velocities, which were deduced by applying 

 to the accelerating potential differences the corrections 

 found in the manner just described. With perfectly pure 

 helium in the apparatus, curves similar to I. of lig. 2 were 

 obtained, indicating that the minimum ionization velocity oE 

 helium is 25'0 volts. 



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15 20 



Electron velocity 



(volts) 



The figure also contains curves showing the effect of a 

 small but gradually increasing quantity of impurity upon 

 the electron velocity at which ionization was first detected. 

 The observations from which these particular curves were 

 plotted were taken with a valve in which the grid and anode 

 were connected together and formed the positive electrode,. 

 the glowing filament being the negative electrode. The 

 pressure of the helium was about 0*85 mm., and the distance 

 of the filament from the grid — about 5 mm. — was several 

 times the mean free path of an electron in the gas, so that 

 when the ionization potential difference was reached, ioni- 

 zation would take place mainly in this space. When the 

 observations plotted in curve I. had been made, a wide-bored 

 tap, which separated the discharge chamber from the puri- 

 fying charcoal tube, was closed, and after an interval of 

 5 minutes, during which the heating of the filament was 

 continued, the characteristic curve was again taken. Curve II. 

 of the figure represents the result obtained. It is seen that 



