244 Mr. G. Owen on the Discharge of 
of potential. The third stage is characterized by the current 
suddenly beginning to increase very rapidly with the poten- 
tial. The first two stages follow directly from the ionic theory 
Diagram 6. 
(200 
{000 
Current. 
Pressure in millimetres. 
of conduction through gases; while the third stage can be 
easily explained on the theory of ionisation by coilision™®. 
This theory assumes that when negative ions move through 
a gas under the action of an electric force they can, under 
certain conditions, produce fresh ions by colliding with the 
molecules of the gas. In order to ionise a molecule the ion 
must possess a definite» kinetic energy. The kinetic energy 
of an ion under a given electric force X is Xel, where ¢ is 
the charge on the ion and / its mean free path. Now / is 
inversely proportional to the pressure. Therefore as we 
diminish the pressure we increase the kinetic energy of the 
ion. When this energy exceeds a certain value fresh ions will 
be produced by collision between the ions and the molecules 
* J. J. Thomson, Proc. Camb. Phil. Soc. Feb. 5, 1900; “ Conduction 
of Electricity through Gases,” pp. 228-234. 
