Closing Years of the Nineteenth Century. 465 



and as the process of conduction in gases came to be better 

 understood, the conductivity produced in the neighbourhood of 

 incandescent metals was attributed to the emission of electrically 

 charged particles by the metals. But it was not until the develop- 

 ment of J. J. Thomson's theory of ionization in gases that notable 

 advances were made. In 1899, Thomson* determined the ratio 

 of the charge to the mass of the resinously charged ions emitted 

 by a hot filament of carbon in rarefied hydrogen, by observing 

 their deflexion in a magnetic field. The value obtained for 

 the ratio was nearly the same as that which he had found for 

 the corpuscles of cathode rays ; whence he concluded that 

 the negative ions emitted by the hot carbon were negative 

 electrons. 



The corresponding investigation-)- for the positive leak from 

 hot bodies yielded the information that the mass of the positive 

 ions is of the same order of magnitude as the mass of material 

 atoms. There are reasons for believing that these ions are 

 produced from gas which has been absorbed by the superficial 

 layer of the metal.J 



If, when a hot metal is emitting ions in a rarefied gas, an 

 electromotive force be established between the metal and a 

 neighbouring electrode, either the positive or the negative ions 

 are urged towards the electrode by the electric field, and a current 

 is thus transmitted through the intervening space. When the 

 metal is at a higher potential than the electrode, the current is 

 carried by the vitreously charged ions : when the electrode is 

 at the higher potential, by those with resinous charges. In 

 either case, it is found that when the electromotive force is 

 increased indefinitely, the current does not increase indefinitely 

 likewise, but acquires a certain " saturation " value. The 

 obvious explanation of this is that the supply of ions available 

 for carrying the current is limited. 



* Phil. Mag. xlviii (1899), p. 547. 



t J. J. Thomson, Proc. Camb. Phil. Soc. xv (1909), p. 64 ; 0. W. Richardson, 

 Phil. Mag. xvi. (1908), p. 740. 



+ Cf. Richardson, Phil. Trans, ccvii (1906), p. 1. 



2 H 



