the dark discharge region of the voltage-current characteristic of a gas discharge tube
When the supply voltage is
zero there is no electric field
and there will no electron flow
between the electrodes of the
discharge tube. This
corresponds with point A of the
characteristic. Despite the
lack of an electric field
however, a limited number of
electrons will still be freed from
their atoms due to the always
present gamma- and cosmic
radiation. When a small supply
voltage is now applied over
the electrodes, a resulting
weak electric field will cause
these electrons to drift into the
direction of the anode. In
point B of the characteristic
this current has reached its
maximum. The number of free
electrons now only depends on the natural background radiation and a further
increase of the voltage causes no further effect until point C. This type of current is
called a dark discharge or Townsend discharge after the scientist John S. Townsend
who profoundly studied this phenomenon.