Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/485

 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- mout of J.J. Thomson's theory of ionization in gages 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.

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.