rrent is produced when the potential
difference is increased; when this stage is reached the current is
constant, and this value of the current is called the "saturation"
value. When the potential difference approaches the value at which
sparks would pass through the gas, the current again increases with the
potential difference; thus the curve representing the relation between
the current and potential difference over very wide ranges of potential
difference has the shape shown in fig. 7; curves of this kind have been
obtained by von Schweidler (_Wien. Ber._, 1899, 108, p. 273), and J. E.
S. Townsend (_Phil. Mag._, 1901 [6], 1, p. 198). We shall discuss later
the causes of the rise in the current with large potential differences,
when we consider ionization by collision.

The general features of the earlier part of the curve are readily
explained on the ionization hypothesis. On this view the Rontgen rays
or other ionizing agent acting on the gas between the plates, produces
positive and negative ions at a definite rate. Let us suppose that q
positive and q negative ions are by this means produced per second
between the plates; these under the electric force will tend to move,
the positive ones to the negative plate, the negative ones to the
positive. Some of these ions will reach the plate, others before
reaching the plate will get so near one of the opposite sign that the
attraction between them will cause them to unite and form an
electrically neutral system; when they do this they end their
existence as ions. The current between the plates is proportional to
the number of ions which reach the plates per second. Now it is
evident that we cannot go on taking more ions out of the gas than are
produced; thus we cannot, when the current is steady, have more than q
positive ions driven to the negative plate per second, and the same
number of negative ions to the positive. If each of the positive ions
carries a charge of e units of positive electricity, and if there is
an equal and opposite charge on each negative ion, then the maximum
amount of electricity which can be given to the plates per second is
qe, and this is equal to the saturation current. Thus if we measure
the saturation current, we get a direct measure of the ionization, and
this does not require us to know the value of any quantity except the
constant charge on the ion. If we attempted to deduce the amount of