as nuclei,
seriously diminish their velocity. If a point is charged up to a high
and rapidly alternating potential, such as can be produced by the
electric oscillations started when a Leyden jar is discharged, then in
hydrogen, nitrogen, ammonia and carbonic acid gas a conductor placed in
the neighbourhood of the point gets a negative charge, while in air and
oxygen it gets a positive one. There are two considerations which are of
importance in connexion with this effect. The first is the velocity of
the ions in the electric field, and the second the ease with which the
ions can give up their charges to the metal point. The greater velocity
of the negative ions would, if the potential were rapidly alternating,
cause an excess of negative ions to be left in the surrounding gas. This
is the case in hydrogen. If, however, the metal had a much greater
tendency to unite with negative than with positive ions, such as we
should expect to be the case in oxygen, this would act in the opposite
direction, and tend to leave an excess of positive ions in the gas.

_The Characteristic Curve for Discharge through Gases._--When a current
of electricity passes through a metallic conductor the relation between
the current and the potential difference is the exceedingly simple one
expressed by Ohm's law; the current is proportional to the potential
difference. When the current passes through a gas there is no such
simple relation. Thus we have already mentioned cases where the current
increased as the potential increased although not in the same
proportion, while as we have seen in certain stages of the arc discharge
the potential difference diminishes as the current increases. Thus the
problem of finding the current which a given battery will produce when
part of the circuit consists of a gas discharge is much more complicated
than when the circuit consists entirely of metallic conductors. If,
however, we measure the potential difference between the electrodes in
the gas when different currents are sent through it, we can plot a
curve, called the "characteristic curve," whose ordinates are the
potential differences between the electrodes in the gas and the
abscissae the corresponding currents. By the aid of this curve we can
calculate the current produced when a given battery is connected up to
the gas by leads of known resistance.

For let E0 be the electromotive force of the battery, R the resistance
of the leads, i the current,