24 Wellisch — Motion of Ions and Electrons through Gases. 



been known that in air at very low pressures the current of 

 negative electricity is due practically entirely to free electrons ; 

 at the higher pressures, however, the current is due to the 

 motion of negative ions. What is the nature of the negative 

 carrier at intermediate pressures ? The answer hitherto given 

 to this question was that the carrier altered in nature during its 

 motion between the electrodes but in such a manner that for a 

 given pressure it possessed an ^average' mass. If, for instance, 

 we regard the ion as being constituted at high pressures by a 

 cluster of molecules, then we would have to assume that as the 

 pressure was reduced the average number of molecules in the 

 cluster decreased ; as the pressure was still further reduced, any 

 individual negative carrier would be for part of the time in the 

 ionic state (say now as a single molecule) and for the remainder 

 would exist as a free electron ; at this pressure we would have 

 at any given instant a number of free electrons and a certain 

 number of ions, but if we were to follow one electron through- 

 out its motion we would find it associated on the average with 

 a mass intermediate between that of an electron and that of a 

 molecule. Ultimately at very low pressures the carriers would 

 be all free electrons. Prof. Townsend's* point of view differed 

 only slightly from this in that he regarded the average nature 

 of the carrier to be determined by electric force as well as gas 

 pressure. 



The answer afforded by the present experiments is funda- 

 mentally different. We now regard the electrons and ions as 

 passing independently through the gas, each kind of carrier 

 remaining constant in nature throughout. The transition from 

 the ionic conduction at high pressures to the electronic con- 

 duction at low pressures is effected by means of an increase in 

 the number of free electrons relative to the number of negative 

 ions without any alteration in the nature of either kind of 

 carrier. The appearance of the phenomenon of ionization by 

 collision would further affect the relative numbers of carriers 

 but would not iniluence the nature of the conduction. 



Looked at from this point of view it seems clear that, as far 

 as the so-called permanent gases are concerned, we must regard 

 the free electrons as occurring theoretically at all pressures. 

 These gases differ, of course, considerably in the relative 

 number of free electrons and ions for any given pressure, and, 

 practically speaking, there is for each gas a pressure at which 

 the number of free electrons is negligibly small but the general 

 rule is in no way invalidated on this account. 



In the above illustration we considered the electric current 

 passing through air. It was shown, however, by Franckf that 



*Townsena, Electricity in Gases, Oxford (1915), Chap. IV, VIII. Cf. also 

 Pidduck, Electricity, Cambridge (1916), Arts. 214-215. 

 f Franck, loc. cit. 



