A gas such as air is normally an almost perfect insulator,
but because of the natural background of radioactive and
cosmic radiation some electrons and ions will always be
present. An applied electric field will, consequently, result
in a current, which under normal conditions and low field
strengths is very feeble. At higher field strengths the current
is greatly increased because of the occurrence of ionizing
processes in the gas and at the surface of the negative
electrode, for air at atmospheric pressure field strengths
higher than 15 kV/cm are required.
The primary ionizing process in the gas is ionization of a
neutral gas molecule by collision with an electron which
has been accelerated by the applied field and thus gained
the necessary energy. By this process a new electron ion
pair is formed. It is a cumulative process and the number
of electrons and positive ions grows extremely rapidly by
the formation of such electron avalanches.
The positive ions are also accelerated in the field, but
they gain considerably less energy than the electrons as
they lose too much energy in each collision because of
their much larger mass, and it is very unlikely that they
can ionize in the gas. They can, however, produce new
electrons by bombardment of the surface of the negative
electrode. Such a process is called a secondary process.
Other important secondary processes are photoelectric
emission from the cathode and photoionization in the
gas caused by photons originating from excited atoms
or from recombination processes.
The growth of the current in a uniform field because of
various primary and secondary processes can be written