1 



produced by the Action of Thorium Compounds. 165 



intensity of the radiation, the total number of ions n produced 

 per second between A and 0, distant d apart, is given by 



n = \ KSIo^-^^, where K is a constant, 



A, 



If e is the charge on an ion, the current i through the gas, 

 when an E.M.F. is applied sufficient to remove all the ions 

 before recombination takes place, is given by 



i = ne. 



Therefore 



SI = 



%K 



Ke(l-e-**) ' 



or the product of the intensity of the radiation and the area 

 of the active surface is proportional to the current through 

 the gas. 



It is of interest to develop the above equation from con- 

 siderations of the energy required to produce an ion. 



Let W be the average amount of energy used up in pro- 

 ducing an ion in the gas. We will assume that the absorption 

 of the energy of the radiation in its passage through the gas 

 is due solely to the production of ions. On account of the 

 absorption, the intensity of the radiation varies from I at the 

 surface of the active plate to I e~ xd at the surface of the top 

 plate. If n be the total number of ions produced, we thus 



0btahl n.W=SI„(l-«-"), 



where the energy absorbed over an area S is given by the 

 right-hand side of the equation ; or 



SI«=- 



w» 



e(l— *-**)' 



where current i=ne, as before. 



Some experiments given in previous papers * point to the 

 conclusion that the energy required to produce an ion may 

 possibly be the same for all gases at all pressures, and it has 

 been shown by Prof. J. J. Thomson and Mr. Townsend that 

 the charge of the ions f in different gases is the same. If 

 such is the case, W/e is a constant for all gases and the current 

 through the gas will depend only on \, d, and SI . 



* Rutherford, Phil. Mag. Jan. 1899. 



t J. J. Thomson, Phil. Mag. Dec. 1898 ; J. S. Townsend, Trans. Roy. 

 Soc. 1899. 



