718 Prof. E. Rutherford and Mr. S. J. Allen : Excited 



This is not very different from the value of 1J» found by 

 Wilson for air inside a silvered glass vessel, using the 

 electroscope method. 



No certain difference was observed in the current for a 

 period of time extending over one month. The production 

 of excited radioactivity from the air suggested the possibility 

 that a radioactive emanation was present in the air and that 

 this might cause the ionization observed. If this is so, the 

 radiating power decays at an extremely slow rate, or the 

 emanation is being continuously reproduced in the inclosed 

 space. 



Application of the Ionization Theory. 



In the spontaneous ionization of air we are dealing with 

 an extremely slow rate of production of ions, and it is of 

 interest to see how far the experimental results are in agree- 

 ment with the ionization theory of gases, which has been 

 previously tested in cases where the ionization is many 

 thousands of times more intense than the present one. We 

 have already noted that the variation of current with the 

 voltage is in general agreement with the theory. 



If q is the constant rate of production of ions per sec. and 

 no electric field is acting, the number n of ions per c.c. 

 increases until the rate of production is equal to the rate of 

 recombination of the ions, or q = an~, where a is the constant 

 of recombination. 



Now we have shown that y = 15, and McOlung* has found 

 from the recombination of ions of Rontffenised air that 



« = 3400e about. 



where e is the charge on an ion. 

 Substituting these values,, we find 



n = 2600, 



i. e., when a steady state is reached, the number of ion- per 

 c.c. is 173 times the number produced per second. The time 

 T taken for this number of ions to diminish to half, supposing 

 the rate of production stopped, is given bv 



= 174 seconds. 



We can obtain a rough approximation of the agreement 

 with theory of the current voltage curve shown in Curve I. 

 (fig. 6) from the following considerations. 



The electric field X, at any point distant r from the centre 

 * Phil. Mao-. March 1902. 



