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D. B. JOY AND A. C. DOWNES
[J. S. M. P. E.
sizes of carbons and the different current values investigated as is clearly shown in Fig. 4.
The areas of the crater openings of the different size carbons at the various currents are given in Fig. 5. The cross-sectional areas of the 9 mm., 13.6 mm., and 16 mm. high intensity carbons are 64 sq. mm., 145 sq. mm., and 201 sq. mm., respectively. It is obvious from the curves that the crater openings for even the higher currents are much less than the original carbon cross-section.
The decrease in crater opening for the lower current densities is
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FIG. 5. Crater opening vs. current.
due in part to the increased spindle or tapering of the portion of the carbon projecting from the positive holder. This increased tapering is due to the enormous decrease in the length of carbon consumed per unit of time for a small decrease in current which allows a longer time for the hot surface of the carbon close to the crater to burn away.
The size of the crater opening or light source of the high intensity carbons is important in considering the application of any optical system for it has long been recognized and clearly demonstrated before this Society11-12 that the light efficiency for motion picture projection decreases rapidly as the area of the light source increases.
The intrinsic brilliancies in candle power per square millimeter of