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Branston carbon arc sunlamp
The usual method to start an arc discharge, was to
temporarily increase the temperature of the
electrodes to a point where thermal emission started
easily. With carbon arc discharge lamps this was
done with contact ignition, which was indeed the
only way to start the arc discharge at all. The
carbon rods were shifted or tilted into such a
position that their tips just touched each other. Due
to the short-circuit current, the rods started to glow
at the point where the electrical resistance was
maximal, the point where the rods touched each
other. When the rods were separated carefully, an
arc discharge developed at the tips of the rods
which then could be moved apart gently until their
desired operational position was reached.
Burning and evaporation of the carbon material,
widened the gap between the rods slowly and the
position of the rods required a re-adjustment from
time to time to prevent the arc from extinguishing.
With carbon arc lamps for illumination purposes this
re-adjusting process often was automated by some
electro-mechanical construction. This kind of
automation was rare with carbon arc sunlamps. Due
to their relatively short treatment- and burning
periods, manual operation with a simple lever of
turning knob, as with the Branston carbon arc
sunlamp on display here, was the standard.
building 4 (technology),
section 5 (ignition circuits)