Ve
α = ---- .
iT
The following table shows the value of α obtained for different gases.
Value of α.
| 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 Owens[[55]]. 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.