DARK-DISCHARGE
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.
