t by the dust particles, the mass of a particle is so great
compared with that of an ion that they are practically immovable under
the action of the electric field, and so the ions clinging to them
escape detection when electrical methods are used. Taking e as 3.5 X
10^-10, we see that [alpha] is about 1.2 X 10^-6, so that the number
of recombinations in unit time between n positive and n negative ions
in unit volume is 1.2 X 10^-6n^2. The kinetic theory of gases shows
that if we have n molecules of air per cubic centimetre, the number of
collisions per second is 1.2 X 10^-10n^2 at a temperature of 0 deg. C.
Thus we see that the number of recombinations between oppositely
charged ions is enormously greater than the number of collisions
between the same number of neutral molecules. We shall see that the
difference in size between the ion and the molecule is not nearly
sufficient to account for the difference between the collisions in the
two cases; the difference is due to the force between the oppositely
charged ions, which drags ions into collisions which but for this
force would have missed each other.

Several methods have been used to measure [alpha]. In one method air,
exposed to some ionizing agent at one end of a long tube, is slowly
sucked through the tube and the saturation current measured at
different points along the tube. These currents are proportional to
the values of n at the place of observation: if we know the distance
of this place from the end of the tube when the gas was ionized and
the velocity of the stream of gas, we can find t in equation (3), and
knowing the value of n we can deduce the value of [alpha] from the
equation

1/n1 - 1/n2 = [alpha](t1 - t2),

where n1, n2 are the values of n at the times t1, t2 respectively. In
this method the tubes ought to be so wide that the loss of ions by
diffusion to the sides of the tube is negligible. There are other
methods which involve the knowledge of the speed with which the ions
move under the action of known electric forces; we shall defer the
consideration of these methods until we have discussed the question of
these speeds.

In measuring the value of [alpha] it should be remembered that the
theory of the methods supposes that the ionization is uniform
throughout the gas. If the total ionization throughout a gas remains
constant, but instead of being uniformly distribute