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The decrease in crater opening for the lower current densities is 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 crater opening have been calculated from the above values of candle power and crater opening and are plotted in Fig. 6. As in the case of the candle power,
the intrinsic brilliancy increases very rapidly as the current is increased on any given size carbon. The values come within the range of those given in the literature1-7. It is believed, however, that this is the first time that data showing the change in intrinsic brilliancy for the currents and sizes of high intensity carbons have been compiled. It is interesting to note that practically the same intrinsic brilliancies are obtained with the various sizes of carbons at the cur
rents 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.13
Typical curves of the spectral energy distribution of the light from the craters of high intensity arcs are given in Fig. 7. The distribution
AN6STPOM Units
Figure Seven
closely approximates that of sunlight.9 The curves show that there is approximately the same amount of energy in the blue region as in the red region for the lower currents on the carbons. As these currents are increased as evidenced by the curves for the 13.6 mm. carbons, the red end of the curve increases faster than the blue so that at the high currents there is actually an appreciable preponderance of red as compared with blue. This is contrary to the distribution curves given in the Bureau of Standards Scientific Paper No. 539, but, as stated previously, the measurements tabulated in that paper were made on the unscreened arc and included the light from the negative arc stream and tail flame which amounts to approximately 32 per cent of the total light and which is known to give (Continued on Page 26)
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