The value of α, which may be termed the _coefficient of recombination_, has been determined in absolute measure by Townsend[52], McClung[53] and Langevin[54] by different experimental methods but with very concordant results. Suppose, for example, with the apparatus of Fig. 6, the time _T_, taken for half the ions to recombine after passing by the electrode _A_, has been determined experimentally. Then 1 --- = α_T_, _N_ where _N_ is the number of ions per c.c. present at _A_. If the saturation current _i_ is determined 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 _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 3500 × _e_ 3492 × _e_ 3400 × _e_ Dioxide 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.