220 Prof. Townsend on the Conductivity produced in 



The distribution of the ionization produced by the rays 

 between the two plates is not quite uniform, which gives rise 

 to small differences in conductivity when the forces are large. 

 The errors arising from this effect are also eliminated by 

 taking the mean of the two conductivities. 



The large difference in conductivity which can be obtained 

 by reversing the electromotive force, when one electrode 

 surrounds the other, is not due to either of the causes men- 

 tioned in this section. The explanation of this result is given 

 in Section 12. 



10. An estimation of the energy required to ionize a 



molecule can be deduced from the results obtained in Section 



7. When an ion collides with a molecule, two new ions are 



produced if the velocity before the impact is sufficiently 



great. It has been shown that new ions are formed when the 



velocity is equal to I 5 . The kinetic energy of an ion moving 



e X 5 

 with this velocity is ^— - =]0 -11 ergs ; and we conclude that 



the energy required to produce two new ions is not greater 

 than this amount. 



An estimation of the energy required to ionize a molecule 

 has also been made by Prof. Rutherford *, and he concluded 

 that the energy required was equal to the work done in 

 moving an ionic charge through 175 volts. If this number 

 is correct, we should not have got any definite increase in 

 the conductivity between two plates by increasing the force, 

 until the difference of potential between the plates con- 

 siderably exceeded 175 volts. It would therefore appear 

 from the present experiments that this estimate of the energy 

 is much too large. 



11. In the short account of the present theory which was 

 published in 'Nature,' I mentioned that the results obtained 

 by Stoletow f, in connexion with the conductivity produced 

 by ultra-violet light, could be explained on the collision 

 theory. We shall examine these results in detail and show 

 that they all agree with our previous conclusions Stoletow 

 investigated the conductivity between, two parallel plates, 

 and found how it varied with the pressure of the air, the 

 distance between the plates, and the electromotive force. 



The following results were obtained by Stoletow : — 

 1. When the pressuie and force were constant, the con- 

 ductivity increased with the distance between the 

 plates. 



* E. Rutherford, Proc. Roy. Soc. vol. Ixvii., 14th Nov. 1900. 

 f Stoletow, Journal de Physique, sene 2, vol. ix. 



