Ions produced hy Rontgen Rays. 67 



electric field immediately applied sufficient to drive over to 

 the electrodes all the ions remaining in the gas. 



By measuring the charge given to the electrode, n was 

 determined. The determination o£ both q and n sufficed to 

 afford an absolute estimation of a. 



Rutherford* and Townsendj made use of another method 

 for verifying the law of recombination. The saturation 

 currents 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 recom- 

 bination a is capable of being expressed in the following 

 form : 



- = 47r(Z; 1 + ^ 2 )e, 

 e 



where e denotes the charge on the ions, h\ and k 2 the ionic 

 mobilities, L e. the velocities with which the positive and 

 negative ions respectively move under unit electrostatic 

 field, and e is a number which is less 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 under the influence of an 

 externally impressed field. The following method was 

 devised by Langevin for the experimental determination 

 of e. The gas between the two parallel plate electrodes was 

 ionized by a single flash of very short duration from a 

 Rontgen-ray bulb ; the ions are regarded as being formed 

 uniformly in layers parallel to the electrodes, although the 

 density of ionization may vary from one layer to another. 

 The charge communicated to the electrodes for electric 

 fields of different intensity is measured by an electrometer; 

 the electric field between the plates is in general chosen 

 very small, so that appreciable recombination may take 

 place during the passage of the ions through the gas. The 



* Loc. cii. 



t Townsend, Phil. Trans. 1899, A, p. 157. 



X Langevin, Ann. de Chim. et de Phys. xxviii. p. 289 (1903). 



F2 



