ionization by measurements of the current before it was saturated, we
should require to know in addition the velocity with which the ions
move under a given electric force, the time that elapses between the
liberation of an ion and its combination with one of the opposite
sign, and the potential difference between the plates. Thus if we wish
to measure the amount of ionization in a gas we should be careful to
see that the current is saturated.

[Illustration: FIG. 7.]

The difference between conduction through gases and through metals is
shown in a striking way when we use potential differences large enough
to produce the saturation current. Suppose we have got a potential
difference between the plates more than sufficient to produce the
saturation current, and let us increase the distance between the plates.
If the gas were to act like a metallic conductor this would diminish the
current, because the greater length would involve a greater resistance
in the circuit. In the case we are considering the separation of the
plates will _increase_ the current, because now there is a larger volume
of gas exposed to the rays; there are therefore more ions produced, and
as the saturation current is proportional to the number of ions the
saturation current is increased. If the potential difference between the
plates were much less than that required to saturate the current, then
increasing the distance would diminish the current; the gas for such
potential differences obeys Ohm's law and the behaviour of the gaseous
resistance is therefore similar to that of a metallic one.

In order to produce the saturation current the electric field must be
strong enough to drive each ion to the electrode before it has time to
enter into combination with one of the opposite sign. Thus when the
plates in the preceding example are far apart, it will take a larger
potential difference to produce this current than when the plates are
close together. The potential difference required to saturate the
current will increase as the square of the distance between the plates,
for if the ions are to be delivered in a given time to the plates their
speed must be proportional to the distance between the plates. But the
speed is proportional to the electric force acting on the ion; hence the
electric force must be proportional to the distance between the plates,
and as in a uniform field the potential difference is equal to the
electric for