Page:Radio-activity.djvu/62

 Value of α.

Gas        Townsend   M^cClung    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} units; thus α = 1·1 × 10^{-6}.

Using this value, it can readily be shown from the equation of recombination that, if 10^6 ions are present per c.c., half of them recombine in about 0·9 sec. and 99% in 90 secs.

M^cClung (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 M^cClung.

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 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 Rutherford, Zeleny , and Langevin 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