Page 9 - RD_2020-09

replaced by the much safer electric arc. But
Mr., Pym was right about one thing
limelight did have a beautiful quality about
it - it was very soft on the eyes
The electric arc using high quality carbon
rods was always popular as a light source
for Motion Pictures. Right from the very
beginning the Lumiere Brothers used a
scissor arc for their first public exhibition
in 1895. Arcs could be operated from
either AC or DC supplies by changing the
arrangement of carbon rods. AC required
two rods of the same diameter - burning to
a point. DC required a larger carbon for the
positive side, which burned to a crater
when the arc was struck; and a smaller
diameter negative carbon which burned to
a point. (The negative had a hard core in
the carbon to produce a point and the
positive had a soft core to produce a
crater.) The carbon gases burning in the
crater produce an intense spot of light that
in early vertical arcs was focused through
a double optical condenser system on to the picture aperture in the projector gate. Some early
systems even used a glass sphere filled with water acting as a condenser to focus the light on to
the gate. The sphere had an open neck at the top to allow the water to expand and release steam
on occasions when the water boiled - the arc is very hot. The big advantage was that the water
kept the gate cooler but the optical characteristics were not as efficient as the double condenser
system.
The DC system had several advantages.
The light was steadier and more intense than AC (at the same voltage and amperage) and was not
subject to pulsing on the screen. (At times the AC supply could synchronise with the projector
shutter causing a strobe effect producing
a pulsing light on the screen.)
A forest of adjusting rods protrude from
the rear of the arc assembly. These
enable the operator -

Left: A hand fed vertical arc assembly. The
carbon diameters vary according to the
voltage and amperage used. 60 volts at 40
amps was common for large screen
presentations. As the carbons burned away
the operator had to feed them almost
constantly to prevent the arc gap from
opening to a point where he lost the arc. (In
the early days he had to crank the projector
at the same time).
September 2020 REEL DEALS 9