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Low Intensity Arc Lamps

The first carbon arc lamps used for cinema projection, as well as in live
theatre follow spotlights and for film studio lighting, were low intensity
arcs. These usually consisted of uncoated black carbons in a near vertical
arrangement as shown in the image on the previous page. A typical trim
would have consisted of 16 mm positive and 10 mm negative carbons
behind a condenser lens which focused the light down towards the gate
in the projector head. The negative carbon was usually aligned so as to
be forward of the centre line of the positive carbon, resulting in a crater
being burned at about a 45 degree angle on the front of the positive carbon
instead of directly on the end of the carbon. In this way, the condenser
lens would collect more light than if the crater were to burn directly on
the end of the positive carbon. The operating current for this type of arc
could be anywhere between 15-80 amps.

High Intensity Arc Lamps
Typical high intensity arc lamp used in larger theatres and drive-Ins.
It was soon realised that vertically aligned, low intensity carbon arc lamps (BTH lamphouse shown).
could be improved by rearranging the carbons such that the positive
carbon was horizontally positioned so as to face the condenser lens, and Rotating Positive Arc Lamps
the negative carbon was directed upwards at an angle towards the end of
the positive carbon. In this arrangement, the crater on the positive carbon As a further refinement to the high intensity carbon arc described in the
could be restricted to the end of the carbon instead of partly up the side previous paragraph, and the need for even higher light outputs for 70 mm
of the carbon. This also meant that the view of the crater, when looking film, drive-in use and even some of the larger cinemas, Strong Electric
from the condenser lens, would not be obstructed by the tip of the commenced manufacture of their rotating positive lamps, where the bare
negative carbon. However, high operating currents were necessary to positive carbon is rotated as it is being fed forwards. Rotation is achieved
produce the light output necessary for the screens in large cinemas. by rolling the carbon between skewered rollers. Current is fed to the
carbon via water-cooled, silver contact blocks located just behind the
actual arc, as opposed to via a clamp at the other end of the carbon. The
negative carbon angling upwards from beneath the mirror was retained,
and still consisted of a copper coated carbon. The mirror diameter varied
from 16 – 18 inches in diameter, and was usually of the dichroic type,
which allowed for much of the heat to pass through the mirror before
being exhausted. Many of these lamps were also fitted with automatic
arc positioning, each carbon having its own feed motor. The operating
current range for this type of lamp was from 75 – 160 amps, the lamp
trim usually consisting of an 11.6 mm black positive and a 9 mm copper
coated negative carbon. So good was the light output from this type of

Hall & Connolly high intensity condenser arc mechanism
In 1926, the Strong Electric Corporation made further improvements by
replacing the front condenser lens with a rear reflector, and aligning the
carbons horizontally with the positive carbon facing the reflector, and
the negative carbon being angled upwards from below the reflector. This
meant that the crater on the positive carbon could be made smaller as it
could now be formed on the end of the carbon instead of partially up the
side. By this means, an arc operating with smaller diameter carbons at a
lower current would produce far more light as well as saving on the cost
of carbons. Lamps operating at up to around 85 amps were built in this
format. The Strong Electric Corporation of Toledo manufactured and
installed the first of this type of lamp in the East Side Auditorium, Toledo,
in 1926.
Above and below: High intensity, rotating positive arc lamp used in larger
Typical high intensity carbon arc used for film projection in smaller theatres. cinemas and later drive-in theatres. (Strong “Futura” lamphouse shown)
(Raycophone lamphouse shown).

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