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PROJECTION ARCS*
A. C. Downes and \V. C. Kalb**
IN THE early years of the motion picture industry large size, low intensity carbons were used for light projection in theatres. Since then the low intensity lamps using large size carbons have been almost entirely replaced by the low intensity reflecting arc lamps, the high intensity reflecting arc lamps, and the high intensity condenser type lamps. In the last few years the need for more light even in the smaller theatres has been augmented by various factors such as the use of perforated sound screens, colored films, and the demand for a higher level of general illumination.
Carbon manufacturers have endeavored to aid the situation in the smaller theatres by recently developing low intensity reflecting arc carbons which carry higher currents than those previously available. It is believed that data on the characteristics of these carbons and their application to projection problems may suggest ways to improve still further the illumination of the screen.
The current carrying capacities of these carbons are given in Table I. Approximately 2 amperes have been allowed as a factor of safety
TABLE
I.
C
urrent
Ritings of S. R. Minimum
A.
Positive Carbons Maximum Current Above
Carbon
Recommended
Recommended Which There is
Diameter
Current
Current Lonsrant Sputtering
10 mm.
21
24 25
12 mm.
28
32 14
1 3 mm.
32
42 44
14 mm.
42
52 55
between the maximum current and the extreme upper limit of the carbons. Data on the 10 and 14 mm. carbons have been included although there has up to this time been no active demand by the industry for these sizes. If the carbons are burned above their maximum current, the light will be unsteady and there will be no appreciable gain in candle-power. If the carbons are burned below the minimum current the efficiency of light production is decreased and the arc tends to become unsteady because the crater area is too small to sufficiently cover the end of the carbon.
It is the light from the crater of the positive carbon which is of value. The light from the negative carbon and arc stream is only approximately 10 per cent of the total light and cannot be utilized for projection purposes. The diameter of the positive crater increases with increasing current. The data in Table II illustrate the change for these particular carbons.
* A resume of recent papers presented before the S.M.I' I ' * National Carbon Company
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