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March, 1930] HlGH INTENSITY ARCS 299
An example of the use that can be made of data of this nature is furnished by comparing the relative light which can be obtained on the screen when 13.6 and 16 millimeter carbons are used with the ordinary plano-convex lens combination. If the 13.6 millimeter carbons were to be burned at 120 amperes and the 1'6 millimeter carbons at 145 amperes, the crater areas (Fig. 5) are 90 and 137 square millimeters and the intrinsic brilliancies are 737 and 620 candle power per square millimeter, respectively. It has been shown in the Transactions12 that for crater areas of 137 square millimeters the relative screen illumination with arc and lenses set properly is approximately 27 per cent more than for a crater area of 90 square millimeters with two sources of the same intrinsic brilliancy. After correcting for the difference in intrinsic brilliancy, it is found that only 7 per cent more light can be expected from the 16 millimeter carbons at 145 amperes than from the 13.6 millimeter carbons at 120 amperes. Such calculations as these, which are made possible in part by the data given above, should be of some assistance in the more efficient use of this very fine source of light for projection of all kinds.
REFERENCES
1 BASSETT: "The High Power Arc in Motion Pictures," Trans. Soc. Mot. Pict. Eng., No. 11 (1920).
2 PRIEST: Tech. Papers Bur. Stand., No. 168 (1920).
3 BASSETT: "Electrochemistry of the High Intensity Arc," Trans. Amer. Electrochem. Soc., 44 (1923).
4 LICHTENBERG: "Military Searchlights," Trans. III. Eng. Soc., 15 (1920).
5 Giu,ETT: "Modern Searchlights," /. Amer. Soc. Naval Eng. (1922).
6 COBLENTZ, DORCAS, AND HUGHES: Sci. Papers Bur. Stand., No. 539 (1926).
7 BENFORD: "High Intensity Arc," Trans. Soc. Mot. Pict. Eng., No. 24 (1925).
8 BASSETT: "Sources of Light," Trans. Soc. Mot. Pict. Eng., 10, No. 27 (1926).
9 GREIDER AND DOWNES: "Sunlight — Natural and Synthetic." A paper presented before the 23rd annual convention of the Illuminating Engineering Society, Philadelphia (Sept., 1929).
10 TOWNSEND: "An Improved Condenser System for Motion Picture Projection," Trans. Soc. Mot. Pict. Eng., 11, No. 31 (1927).
11 STORY: "Preliminary Measurements of Illumination in Motion Picture Projection," Trans. Soc. Mot. Pict. Eng., No. 9 (1919).
12 STORY: "Further Measurements of Illumination in Motion Picture Projection," Trans. Soc. Mot. Pict. Eng., No. 10 (1920).
13 CADY AND DATES: Illuminating Engineering, Second Edition, John Wiley and Sons, Inc., New York (1928), pp. 38 and 76.
DISCUSSION
MR. STOLLER: I should like to ask Mr. Downes what prospects there are for increasing the amount of illumination available. With the advent of color