S. J. Plimpton — Recombination of Ions. 39 



Art. VI. — The Recombination of Ions Produced by Ront- 

 gen Rays ; by S. J. Plimpton, Ph.D. 



When a gas is exposed to the continued action of Rontgen 

 rays the number of ions contained in a unit volume of the gas 

 does not go on increasing indefinitely because of the electro- 

 static field associated with the ions. A continuous diffusion 

 takes place from the region of maximum ionization and in 

 addition oppositely charged ions recombine under their attrac- 

 tive forces. The process of diffusion is a relatively slow one, 

 and with properly designed apparatus, may be disregarded in 

 measuring the changes in ionization due to recombination. 



According to the accepted theory of ionization in gases the 

 rate of change of the number of ions of either sign per unit 

 volume is proportional to the square of this number, i. e. 



^ =■--»■ 0) 



where a is a constant known as the coefficient of recombina- 

 tion and is independent of n. 



This law of recombination has been shown by several experi- 

 menters to hold good to a high degree of approximation. 



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 con- 

 nected with a source of potential and the other with an elec- 

 trometer system. 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 recombina- 

 tion ; 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 provided 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 x 



which is obtained immediately by integrating equation (1), n 

 being the number of ions per cubic centimeter 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. 



* Rutherford, Phil. Mag., xliv, p. 422, 1897. 

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



