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PROJECTION ARCS
13!
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 that the light efficiency for motion picture projection decreases rapidly as the area of the light source increases.
160
National High Intensity Carbons
CRATED OPENING
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80 90 100 110 120
CURRENT
Fig. 8. Crater opening vs. current.
The intrinsic brilliancies in candle power per square millimeter of crater opening are plotted in Fig. 9.
As in the case of the candle power, the intrinsic brilliancy increases very rapidly as the current is increased on any given size carbon. It is interesting to note that practically the same intrinsic brilliancies are obtained with the various sizes of carbons at the currents ordinarily used. These values, ranging from 500 to 750 candle power per square millimeter, illustrate quite forcibly the advantage that the high intensity arc has for projection purposes over the plain carbon arc with an intrinsic brilliancy of 130 candle power per square millimeter and the incandescent tungsten filament projector lamp run at overvoltage with an intrinsic brilliancy of 27 candle power per square millimeter.
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 16 millimeter