﻿848 



Ionization by Collision in Helium. 

 Table XL 

 Helium. p = 5 mm. X = 200. 





K. 



|3. 



a'. 



V. 



Pure Helium i .. 



Ileliura 4- '25 p.c. air 



Helium +'5 p.c. air 



Helium +'75 p.c. air 



5-4 

 6-1 



5-0 

 5-0 



•082 

 •069 



•065 

 •051 



"78 

 •85 

 •88 

 •93 



156 

 170 

 176 



187 



General Discussion of Results. 



The last table showsvery clearly the effect of impurities upon 

 the ionizing powers of the ions. As far as a is concerned the 

 effect of 3 parts in 400 of air is to reduce the value o£ a by 

 about 8 per cent., but the effect upon ft is very much greater, 

 reducing it from *082 to *051, that is by nearly 40 per cent. 

 The other two values of ft are not necessarily very accurate, 

 as they were not tested by passing a spark for fear of adding 

 more impurities to the gas, but the value '051 was thus tested 

 and found correct, as was the value '082 in the course of the 

 experiments given in Table II. 



A possible explanation for the fact that the impurities have 

 a much greater effect upon the positive than upon the 

 negative ions, is that they more readily attach themselves to 

 the slow moving positives than to the much faster negatives; 

 the positives thus loaded are less effective in producing fresh 

 ions by collision, because they are longer in reaching the 

 critical velocity necessary for ionization. Experiments upon 

 the velocities of the positive ions in helium should give inter- 

 esting results. The reduction of ft makes a very great 

 difference to the sparking potential, less than 1 per cent, of 

 impurity bringing it from 156 to 187 volts. 



The minimum sparking potential attained of 156 volts is 

 far below that observed before for any other gas ; those of 

 nitrogen 298 volts, argon 233 volts, and hydrogen 273 volts, 

 being the three lowest previously obtained. 



a X 



A formula can be obtained for - in terms of — bv making 



p p J to 



simple assumptions as to the effects of collisions on the velocity 

 of the ions *.. 



* J. S. Townsend, ' Theory of Ionization of Gases by Collision,' 

 p. 23. 



