66 Dr. S. J. Plimpton on the Recombination of 



This law of recombination has been shown by several 

 experimenters to hold good to a high degree of approxi- 

 mation. 



In the early experiments of Rutherford* and of McClungf 

 the gas was confined in a closed metal chamber provided with 

 an aluminium window for admitting the rays. Inside the 

 vessel were two parallel metal plates, one of which was 

 connected with a source of potential and the other with an 

 electrometer. 



Under the action of the rays the gas between the electrodes 

 was ionized until a steady state was attained, when the 

 ionization was balanced by diffusion and recombination ; 

 the rays were then switched off, and the charge remaining 

 in the gas at definite times after the cessation of the rays 

 was then determined. A pendulum interrupter was pro- 

 vided, so that this time-interval could be varied at will. In 

 this manner corresponding values of n and t were obtained, 

 and were found to agree satisfactorily with the relation 



--- = at, (2) 



n n 



which is obtained immediately by integrating equation (1), 

 n being the number of ions per cubic centimetre present in 

 the gas when the steady state has been established, and 

 n being the corresponding number after the ions have been 

 allowed to recombine for a time t. 



Another method was also employed by McGlung % to 

 demonstrate the validity of the law of recombination, and 

 was used by him to determine an absolute value of the 

 coefficient a. The saturation current was measured while 

 the rays were acting, a strong electric field being maintained 

 between the electrodes. From this observation the number 

 of ions (q) produced per cubic centimetre per second in the 

 gas was determined in arbitrary units. The electric field 

 was then withdrawn, and the rays were allowed to act until 

 a steady state of ionization was produced ; this is expressed 

 by the equation __ 2 x«n 



where n denotes the number of ions of either sign present 

 per cubic centimetre in the gas when the steady state has 

 been reached. The rays were then cut off and a strong 



* Kutherford, Phil. Mag. xliv. p. 422 (1897). 

 t McClung, Phil. Mag. xiii. p. 283 (1902). 

 % Loc. cit. 



