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March, 1930]
HIGH INTENSITY ARCS
297
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 literature.1'7 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
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CURRENT
FIG. 6. Intrinsic brilliancy vs. current.
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.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 closely approximates that of sunlight.9 The curves show that there is approximately the same amount of energy in the blue region as in