at the electrode A, i = NVe, where e is the charge on an ion and V is the volume of uniformly ionized gas carried by the electrode A per second. Then
The following table shows the value of α obtained for different gases.
| Gas | Townsend | McClung | Langevin |
|---|---|---|---|
| Air | 3420 × e | 3384 × e | 3200 × e |
| Carbon Dioxide | 3500 × e | 3492 × e | 3400 × e |
| Hydrogen | 3020 × e |
The latest determination of the value of e (see section 36) is 3·4 × 10-10 E.S. units; thus α = 1·1 × 10-6.
Using this value, it can readily be shown from the equation of recombination that, if 106 ions are present per c.c., half of them recombine in about 0·9 sec. and 99% in 90 secs.
McClung (loc. cit.) showed that the value of α was approximately independent of the pressure between ·125 and three atmospheres. In later observations, Langevin has found that the value of α decreases rapidly when the pressure is lowered below the limits used by McClung.
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 Owens12. 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 conditions. A much larger voltage is then necessary to produce saturation. If the smoke particles are removed by a stream of air, the current returns at once to its original value.
32. Mobility of the ions. Determinations of the mobility of the ions, i.e. the velocity of the ions under a potential gradient of 1 volt per cm., have been made by Rutherford13, Zeleny14, and Langevin15 for gases exposed to Röntgen 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 velocity proportional to the strength of the field. On the application of an electric field, the ions almost instantly attain the velocity corresponding to the field and then move with a uniform speed.
Zeleny16 first drew attention to