i] IONIZATION THEORY OF GASES 39 
Value of a. 
Gas Townsend MeClung Langevin 
AT Tae heen ues 3420 x e 3384 x e 3200 x e 
Carbon Dioxide 3500 x e 3492 xe 3400 x e 
Hydrogen ... 3020 xe 
The latest determination of the value of e (see section 36) is 
34x 10-" Es. units; thus a=11 x 10™. 
Using this value, it can readily be shown from the equation of 
recombination that, if 10° ions are present per c.c., half of them 
recombine in about 0°9 sec. and 99°/, in 90 secs. 
M*Clung (Joc. cit.) showed that the value of a was approximately 
independent of the pressure between ‘125 and three atmospheres. 
In later observations, Langevin has found that the value of a 
decreases rapidly when the pressure is lowered below the limits 
used by M°Clung. 
31. In experiments on recombination it is essential that the 
gas should be free from dust or other suspended particles. In 
dusty air, the rate of recombination is much more rapid than in 
dust-free air, as the ions diffuse rapidly to the comparatively large 
dust particles distributed throughout the gas. The effect of the 
suspension of small particles in a conducting gas is very well 
illustrated by an experiment of Owens!. If tobacco smoke is 
blown between two parallel plates as in Fig. 1, the current at once 
diminishes to a small fraction of its former value, although a P.D. 
is applied sufficient to produce saturation under ordinary con- 
ditions. A much larger voltage is then necessary to produce 
saturation. If the smoke particles are removed by a stream of air, 
the current at once returns to its original value. 
32. Mobility of the ions. Determinations of the mobility 
of the ions, z.e. the velocity of the ions under a potential gradient 
of 1 volt per cm., have been made by Rutherford?, Zeleny®, and 
Langevin* for gases exposed to Réntgen rays. Although widely 
different methods have been employed, the results have been very 
concordant and fully support the view that the ions move with a 
1 Phil. Mag. Oct. 1899. 2 Phil. Mag. p. 429, Nov. 1897. 
3 Phil. Trans. A, p. 193, 1901. 4 C. R. 134, p. 646, 1962. 
