368 Prof. J. J. Thomson on easily Absorbed 



leak rises very rapidly to a maximum and then falls more 

 slowly. It is apparent at a distance from the cathode which is 

 a large multiple of the thickness of the dark space. The 

 greater the current through the discharge-tube the greater is 

 the rate of leak, and the further away from the cathode can 

 it be detected. The greater the current, the greater is the 

 extension of the negative glow. 



As the radiation which produces the electricity from the 

 disk shows all the characteristics of Rontgen radiation, 

 although it has extraordinary small penetrating power, it 

 seems legitimate to infer that it arises from the impact 

 against the windows of the comparatively slowly moving 

 corpuscles in the negative glow. The intensity of the 

 radiation is a measure of the rate at which the energy due to 

 these corpuscles is passing unit area at the place against 

 which they strike. The numbers just given show that this 

 rate is small inside the cathode dark space, rapidly increasing 

 to a maximum at its luminous boundary, and then diminishing 

 practically to zero at the positive end of the negative glow. 

 This result is readily explained by the view I put forward in 

 a paper read before the Cambridge Philosophical Society in 

 February 1900 (see Phil. Mag. 1. p. 278). On this view, 

 the ionization which occurs when the electric discharge 

 passes through a gas is due to the motion of other ions 

 through the gas ; thus each ion is, as it were, the child of 

 another ion, the parent having been set in rapid motion by 

 the electric field. The ionization by cathode and Lenard 

 ra} r s is a particular case of this principle. The view is further 

 strengthened by the experiments recently made by Professor 

 Townsend in the Cavendish Laboratory (Phil. Mag. Feb. 

 1901) on the ionization produced by the motion under an 

 electric field through gases of low pressure of negative 

 corpuscles produced by Rontgen rays : according to these 

 experiments the negative corpuscle is a much more efficient 

 ionizing agent in an electric field than the positive ion. 



I shall now proceed to give a more detailed explanation of 

 the phenomena in vacuum-tubes from this point of view than 

 I gave in my earlier paper. 



Let us first consider the number of ions produced per second 

 in unit volume of the gas. These ions are produced by the 

 collision of the corpuscles with the molecules of the gas ; 

 and inasmuch as to ionize a molecule requires a finite amount 

 of energy, if the energy of the corpuscles does not exceed a 

 certain value, there will not be any ionization, while if the 

 energy exceeds this critical value, then in a certain fraction 

 of the collisions ionization will take place, this fraction 

 increasing with the kinetic energy possessed by the particles. 



