40 S. J. Plimpton — Recombination of Ions 



Another method was also employed by McClung* to demon- 

 strate 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 act- 

 ing, a strong electric field being maintained across the elec- 

 trodes. From this observation the number of ions (q) 

 produced per cubic centimeter 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 withdrawn and the rays were allowed to act until a steady 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 represented by the 

 equation 



q = an 2 (3) 



where n denotes the number of ions of either sign present per 

 cubic centimeter in the gas when the steady state has been 

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

 field immediately applied sufficient to drive over to the elec- 

 trodes all the ions remaining in the gas. By measuring the 

 charge given to the electrodes n was determined. The deter- 

 mination of both q and n sufficed to afford an absolute estima- 

 tion of a. 



Kutherfordf and Townsend^: made use of another method 

 for verifying the law of recombination. The saturation cur- 

 rents were measured at different parts of a tube through which 

 ionized air was streaming. These currents give the values of 

 n at the place of observation, and if the rate of flow of the air 

 is determined and the distance from the region where the ions 

 are formed is known, the series of values of n corresponding 

 to different time intervals can be plotted. This method can be 

 employed only when the gas used is available in large quantities. 



Langevin§ has shown that the coefficient of recombination a 

 is capable of being expressed in the following form 



\ = M*i + K)* 



where e denotes the charge on the ions, h 1 and Jc 2 the ionic 

 mobilities, i. e. the velocities with which the positive and nega- 

 tive ions respectively move under unit electrostatic field, and e 

 is a number which is ]ess than unity. According to the theory 

 of Langevin, e represents the ratio of the number of collisions 

 between oppositely charged ions which result in recombination 

 to the total number of collisions ; in this theory it is assumed 

 that the ions move under their mutual attractions following 

 the same law and with the same mobilities as if they moved 



* Loc. cit. + Loc. cit. 



JTownsend, Phil. Trans.. A, p. 157, 1899. 



§ Langevin, Ann. Chim. Phys., xxviii, p. 289, 1903. 



