Cooper Hewitt saturation lamp
LOW-PRESSURE-ARC-DISCHARGE
Under low-pressure conditions the
ionisations within the gas column are
mainly caused by collisions of free
electrons with gas atoms. The
temperature of these free electrons can
be very high but since the mass of an
atom is much greater than that of an
electron and because only a limited part
of the atoms will get ionised, the mean
gas temperature will rise only a little. An
example of such a low-pressure mercury
vapour arc discharge could be found in
sunlamps of the Cooper Hewitt type in
which the cathode consisted of a
relatively large pool of mercury in a
vacuum tube. The arc discharge was
established between the anode and a hot
spot at the surface of the cathode pool.
The mercury vaporised at the hot spot
and condensed in the cooler anode part
of the tube from where it flowed back into
the pool of mercury. Because of the
temperature dependency of the gas
pressure a proper cooling of the tube was essential for keeping the pressure low. The
electric current was limited to such a level that only a part of the mercury would
vaporise. Since the atmosphere within the gas discharge tube would become
saturated with mercury vapour, this type of ultraviolet radiator was often determinated
as a saturation lamp.
