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} E.S. 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[1]. 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 Rutherford[2], Zeleny[3], and
Langevin[4] 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
