The relative spectral radiance of the low-current carbon arc, which includes contributions from both the incandescent anode and the arc stream through which it is viewed, was measured from 1900 A to 2500 A and placed on an absolute scale by comparison with measurements made at longer wavelengths. The curve bears no resemblance to a blackbody curve in this spectral region, but has a minimum near 2400 A, increasing slowly to shorter wavelengths. The values near 2000 A are a little higher than those near 2500 A. The spectral radiance of the arc stream alone was also measured. A grating monochromator was used in the work, and the speed of the diffraction grating was measured separately by placing it in the exit beam of another grating monochromator. The receiver was a 1P21 photomultiplier sensitized for the ultraviolet by placing on its envelope a thick coating of sodium salicylate, a fluorescent material which possesses constant quantum efficiency for exciting wavelengths throughout this region.
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