acted to
plate _B_, while those negatively charged are attracted to plate _A_,
thus causing a current. These ions will recombine and neutralize their
charges if the opportunity is given. The number, therefore, increases
to a point at which the ions produced balance the number recombining.

When an electric field is produced between the plates, the velocity of
the ions between the plates is increased in proportion to the strength
of the electric field. In a weak field the ions travel so slowly that
most of them recombine on the way and consequently the observed
current is very small. On increasing the voltage the speed of the ions
is increased, fewer recombine, the current increases, and, when the
condition for recombination is practically removed, it will have a
maximum value. This maximum current is called the saturation current
and the value of the potential difference required to give this
maximum current is called the saturation P.D. or saturation voltage.

The picture, then, is this. The radiations separate the components of
the gas into ions, or carriers of electricity, half of which are
charged negatively and half positively. In the electric field those
negatively charged seek the positive plate and those positively
charged seek the negative plate. If time is given, these ions meet and
recombine, their charges are neutralized, and there is no current.

Experimental Confirmation

This theory of the ionization of gases has been most interestingly
confirmed by direct experiment. For instance, the ions may form nuclei
for the condensation of water, and in this way the existence of the
separate ions in the gas may be shown and the number present actually
counted.

When air saturated with water vapor is allowed to expand suddenly, the
water present forms a mist of small globules. There are always small
dust particles in air and around these as nuclei the drops are formed.
These drops will settle and thus by repeated small expansions all dust
nuclei may be removed and no mist or cloud will be formed by further
expansions.

If now the radiation from a radio-active body be introduced into the
condensation vessel, a new cloud is produced in which the water drops
are finer and more numerous according to the intensity of the rays. On
passing a strong beam of light through the condensation chamber, the
drops can readily be seen. These drops form on the ions produced by
the radiation.

Application of Electric Fiel