26 Wellisck — Motion of Ions and Electrons through Gases. 



tro-negative atoms such as bromine, iodine, etc. This statement 

 is to be regarded merely as a suggestion for fnrtiier experi- 

 ments ; a study of the ionization in pure ammonia might prove 

 of interest in this connection. 



It is known that tlie presence of a trace of oxygen in an 

 inert gas or in hydrogen at atmospheric pressure will reduce 

 considerably the number of free electrons. The present ex- 

 periments showed that in hydrogen the sensitivity of the free 

 electrons to traces of oxygen was greatly decreased if the gas 

 pressure was reduced so that, for instance, a considerable num- 

 ber of free electrons was obtained in a mixture of hydrogen 

 at 82J-™"' pressure and air at 2|-'"™. In a previous communica- 

 tion"^ a definite theory in explanation of these results has been 

 given ; the underlying idea is that an electron cannot effect a 

 permanent union with an uncharged molecule to form a nega- 

 tive ion unless the relative velocity at collision exceed a critical 

 value characteristic of the molecule concerned. We have seen 

 that in a large number of gases the electrons persist in the free 

 state so that it would appear that the negative ions in these 

 gases must in generalf be formed immediately after the act of 

 ionization. We may regard the electron as being expelled 

 with a certain velocity from an uncharged molecule, but owing 

 to the positive charge acquired by the molecule the velocity of 

 the electron will decrease as it recedes ; in accordance w^ith the 

 above view we may imagine a sphere drawn round the parent 

 molecule of such a radius that the electron will be effective in 

 forming a negative ion only for impacts within this sphere. 

 It is probable that the circumstances of an encounter as well 

 as the relative velocity will determine the effectiveness of a. 

 collision so that only a fraction of these impacts will result in 

 the formation of ions ; outside the sphere, however, the elec- 

 tron must continue in the free state. It is easy to see that on 

 this view the relative number of electrons will increase with 

 decreasing pressure. 



The potential required for the formation of a negative ion 

 must of course be less than that required to ionize a molecule 

 inasmuch as in the latter case a fresh pair of ions originates. 

 We would expect that for those gases which have a high 

 ionization potential the proportion of negative ions to electrons 

 would in general be small. This is borne out by the results 

 for the inert gases and hydrogen, although the value assigned 

 by Franck and Hertz;}: for the ionization potential in nitrogen 

 (viz. 7*5 volts) would not indicate on this view a very large 

 percentage of free electrons. 



*Wellisch, Phil. Mag., vol. xxxl, p. 186, 1916. 



f When the applied field is sufficiently great to generate the critical 

 velocity in the electron, negative ions will again commence to be formed, 

 :}: Franck and Hertz, Verh. Deutsch. Phys. Ges., xv, p. 34, 1913. 



