﻿Ionization hy Collision in Helium. 845 



The values obtained are exhibited in the following 

 table :— 



Table VII. 



X 



« 



1 



p 



V 



P 



5 



•126 





10 



•27 





20 



•57 





38 



103 



•0135 



40 



1-08 



•0164 



80 



1-83 



•040 



120 



2-1 



•070 



200 



2-37 



•165 



The curves obtained in 1908 for helium with an estimated 

 impurity of about 2 per cent, are also put in the figs. 2 and 3, 

 showing that both u and /3 are diminished by a trace of 

 impurity. 



The Sparking Potentials. 



The values of a and ft given above determine the sparking 

 distance a for the various pressures and electric forces from 

 the formula a— fte^ a ~^ a = 0. This was verified by gradually 

 increasing the distance between the plates, keeping the electric 

 force X per cm. constant; the distances a' at which the spark 

 first appeared together with the corresponding sparking 

 potential a'X are given below, the agreement between a and 

 a' is very good. 



Table VIII. 



X. 



p. 



a. 



a'. 



V. 



240 



2 



•84 



•81 



194 



240 



3 



•71 



•71 



170 



200 



5 



•79 



•78 



156 



198 



52 



•82 



•83 



164 



