for Measuring Ionization. 713 



screening. The well-known " rate of leak " method in radio- 

 activity has served admirably for qualitative experiments, 

 but when used for accurate quantitative results it has several 

 objections. The rate of movement of the needle is not 

 always uniform, but is liable to sudden variations due to 

 various causes, principal among which are static charges in 

 the neighbourhood. 



When used for strong ionizations the movement may be 

 too rajoid to be read accurately with a stop-watch, neces- 

 sitating either a change of sensitiveness of the needle, 

 or an adding of capacities in parallel with the charging 

 quadrants. 



When it is desired to measure the ionization due to a 

 rapidly decaying radioactive substance, the " rate of leak " 

 method is no longer applicable, since the time taken to make 

 a reading is too large compared with the rate of decay of 

 the activity. 



In order to overcome this difficulty, Bronson * has devised 

 a very convenient method in which the steady deflexion of 

 the needle is proportional to the instantaneous value of the 

 activity. This is brought about by connecting in parallel 

 with the charging quadrants a pair of parallel metal plates, 

 the lower one being connected to earth and containing a 

 strong radioactive material such as polonium. The ioniza- 

 tion between the plates converts the air into a high 

 resistance over which the accumulating electricity leaks to 

 the earth. 



When the leak of electricity away is just balanced by the 

 supply, the needle will take up a position of rest proportional 

 to the intensity of ionization in the testing-vessel at that 

 instant. The difference of potential between the plates of the 

 44 air " resistance is of course very low, and the leak there is 

 mostly due to a diffusion of the ions, and not to a saturation 

 effect. 



The method used by the author is somewhat similar in 

 principle to that of Bronson, and may have been used before 

 in practice, though he has never seen any account of it in 

 print. The general idea is briefly this. 



The ionization which it is desired to measure is balanced 

 against that due to a standard, previously accurately cali- 

 brated, the electrometer simply serving as a means of telling 

 when this balance has been obtained. 



The details of the method can be readily understood by 



* Bronson, Amer. Jonrn. Sci. Feb. 1905. 



