THEORY OF ELECTRIC DISCHARGES 83 



some of which are empirical and do not aim at giving 

 an explanation of the fundamental principles on which 

 the potentials depend, and others, depending on various 

 assumptions which attribute certain properties to posi- 

 tive and negative ions, appear to explain more or less 

 accurately some of the phenomena which are observed. 

 Professor Sir J. J. Thomson has recently given two 

 theories of the latter kind, and it is interesting to 

 examine how far they furnish results which account for 

 the various potentials and to what extent the assumptions 

 made in the process of the investigations agree with 

 those properties of ions which are found to accord with 

 simpler phenomena. 



The first 1 of these investigations differs essentially 

 from that given in section 20 above, and deals with the 

 problem of finding the potential required to maintain a 

 current flowing between electrodes when the ordinary 

 cathode fall of potential is established. No distinction 

 is drawn between the sparking potential and the poten- 

 tial required to maintain a current. It is obvious that 

 the expression which is thus found cannot represent the 

 sparking potential, and the apparent agreement between 

 the formula and the sparking potentials obtained experi- 

 mentally results only from attributing convenient values 

 to undetermined constants. Clearly the formula for the 

 potential that is found in this way should represent the 

 potential required to maintain a comparatively large 

 current, if the assumptions on which the theory is based 

 are correct. 



The current on this theory is kept up by the ionization 

 of the molecules of the gas by negative ions, and* a 



1 J. J. Thomson, "Conduction of Electricity through Gases," 

 1903, p. 38. 



