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Limelight. mixture required an even hotter flame REFERENCES:
It was the title of a Chaplin film and than did the lime and it yielded a light The Cinematograph Book. Bernard E
a key element in ‘The Picture Show more powerful than limelight, although Jones, Revised Edition 1921. Castle
Man’, but what exactly was it? Ross it was still called by the same name. and Co.
The Modern Bioscope Operator.
King explains the science behind an The size of the acetylene generator
Ganes Ltd, 1911
evocative word entrenched in our was governed by the number of hours
language. the lights were to burn and the candle-
From the earliest days of the power of the burners. Once the pure-
cinematograph the electric arc was the oxygen/acetylene gas mixture and
best of all illuminants where there was pressure was correctly set, the
an electricity supply. Generated illuminant rarely required further
electricity however, was only available adjustment during a film show.
in the larger towns and cities, and Acetylene-generated limelight was
outlying areas and country towns relied the common form used with the
on oil lamps and coal gas for lighting. cinematograph, even though it involved
Purpose-built picture theatres began the compression of acetylene and was
to appear in the cities from about 1910 not considered safe in the hands of any
and most of them used arc lamps. but those practised in the work.
However, before Edison developed the Acetylene when compressed is
electric arc lamp the only method of liable to explode by simple shock. To
producing a bright light source was reduce the risk of explosion known as a
limelight. The term ‘in the limelight’ ‘snap,’ the acetylene was compressed
originated from the stage where into liquid acetone, a laboratory and
performers were ‘lit’ using this process. industrial solvent.
For travelling picture-show men Diagram (1) shows the cross section
transporting their equipment by horse of a simple acetylene generator.
drawn wagon to small towns and Oxy-acetylene limelight
settlements where electricity supplies The combustion of acetylene with
were non-existent, the only practical oxygen, under pressure, as in the
means to project films to a paying blowpipe or limelight jet, produces a
audience was to use limelight. It was an temperature in the hottest part of the
excellent illuminant for the hand- flame of about 4,000 degrees C.,
cranked cinematograph projector, but probably twice that reached in the oxy-
was not without its safety hazards. hydrogen or oxy-coal-gas flame.
Limelight was produced by playing Lime melts at approximately 3,000
an intense flame upon a cylinder of lime, degrees C., too low a temperature to be
the heat raising a spot of lime to brilliant useful, so the ‘pastille’ of rare earths
incandescence. The three elements was used.
required to produce limelight were:-
Both the acetylene and the oxygen
Lime. Cylinders or rare earth must be under pressure, and about
pastilles stored in air-tight containers to equal quantities of each were used;
prevent premature atmospheric either produced by generators or taken
deterioration. from commercial pressurised cylinders.
Gas. Principally regulated acetylene In a generic oxyacetylene plant the
but hydrogen, ether and petrol were oxygen is generated with oxygenite
also used. powder (one of the sodium peroxide
Oxygen. Pure oxygen regulated preparations) and the acetylene is
under pressure. produced by a generator. As an
Lime subjected to an ordinary gas example, one charge (30 oz of
flame becomes dull red-hot; but when oxygenite) is sufficient for 90 minutes
combustion is forced by mixing pure- of light, at the rate of 6 cubic ft. of
oxygen with the gas, the flame becomes oxygen per hour.
hotter and more concentrated, and the The combination of limelight and
lime becomes brilliantly incandescent. its oxyacetylene gas supply, along with Generating limelight.
Top: Cross section of acetylene
Acetylene is a gas evolved by the highly flammable nitrate film stock,
generator.
action of water on calcium carbide. In made for hazardous working conditions
Centre: One brand of generator.
one form of generator, water drips upon for the showman/projectionist.
Above: Detail of burner jet using pastilles.
the carbide, and in the other small Although there were fires, they never
lumps of carbide fall into water. A later resulted in great loss of life in
advance replaced the lime cylinder with Australia, unlike the experiences in
round ‘pastilles’ made from the rare other parts of the world during those
earths-thorium and cerium. This early years of the cinema. ★

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