178 Prof. E. Rutherford on Radioactivity 



if i = initial current, it is clear that 



Q=£. 



In the case given in the last table, the initial current was 

 1-6 x lO" 11 amperes, and the value of L = '00001 89; therefore 

 the total quantity of electricity passing between the cylinders 

 is equal to 8'5 x 10 -7 coulombs. 



The total quantity of electricity separated, if the radiation 

 has been completely absorbed in the gas, is obviously 



of this quantity. 



In the above case, a = '20 cm., b = o'65 cms., \=-42. 



Therefore quantity passing between cylinders =11*1 x 10~ 7 

 coulombs. 



Increase of Induced Radioactivity with Time. 



If a plate or wire is exposed to the action of thorium oxide 

 in a closed vessel, the radioactivity at first increases nearly 

 proportionally with the time, and then more slowly, finally 

 tending to a maximum value after several days' exposure. 



The table given below is an example of the results obtained 

 for a square zinc plate, area 86 sq. cms., exposed in a metal 

 vessel, with a potential-difference of 300 volts between thorium 

 and surface to be made active. The plate was removed from 

 the action of the thorium at intervals for sufficient time to 

 determine the current produced by it between two charged 

 parallel plates, as in fig. 1. 



Time of exposure Q 



in hours. 



1-58 '063 



3-25 -105 



5-83 -289 



9-83 -398 



14-00 -586 



23-41 -773 



29-83 -834 



47-00 -898 



72-50 -951 



96-00 1-00 



The current after four days' exposure is taken as unity, as 

 the rate of leak had nearly reached its maximum value. 



The maximum value of the current produced bv the active 



