. Instruments et méthodes utilisés en radiométrie, III - la cellule photoélectrique et autres radiomètres sélectifs . etin du Bureau des normes [vd. 14 pour les étoiles binaires ayant des étoiles compagnes qui ne sont pas sufiicientlyluminoi:s pour être visible à l'oeil. II LA CELLULE PHOTOÉLECTRIQUE les récents progrès dans la construction de cellules photoélectriques, en particulier les cellules très sensibles de Kunz, dans lesquelles l'activétérial est l'hydrure de potassium, semblent justifier leur utilisation comme radiomètre de pré-ciision pour une classe limitée d'investigations, comme, par exemple, la transmission et les spectres de réflexion dans cette partie de
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. Instruments and methods used in radiometry, III - The photoelectric cell and other selective radiometers . etin of the Bureau of Standards [vd. 14 for binary stars having companion stars which are not sufiicientlyluminoi:s to be visible to the eye. II. THE PHOTOELECTRIC CELL Recent advances in the construction of photoelectric cells, especially the highly sensitive cells of Kunz, in which the activematerial is potasssium hydride, seem to warrant their use as a pre-cision radiometer for a limited class of investigations, such as, forexample, transmission and reflection spectra in that part of thespectrum extending from the yellowish green into the extremeultra-violet. Such investigations have already been made.^ Instellar photometry important results have been obtained byStebbins ® in the measurement of the variation in brightness ofvariable stars, w^hile numerous other applications are being made.How^ever, before undertaking such applications the cell must bethoroughly tested for direct proportionality between the intensityof the incident light and the resulting photoelectric current. This. —cFig. 1.—Photoelectric cell {Kunz) may be accomplished by changing the distance of the light, byusing Nicol prisms, or by means of a sectored disk, as describedelsewhere in this paper. In stellar radiometry its greatest usefulness will be in studyingvariable stars which do not change in color. (a) Description of Cell.—Various designs of photoelectric cellshave been suggested. Kunz ^^ has described the latest develop-ments which appear to give a cell which is quite free from darkciirrents; that is, leakage currents flowing when the cell is notexposed to light. The design is illustrated in Fig. i. It may beconstructed of quartz or of glass or of glass with a quartz window.The central bulb is about 3.5 cm in diameter. The cathode, C, is of small platinum wire, and this part of the bulb is silvered to 8 Hulburt, Astrophys. Jour., 42, p. 205, 1915; and Nathanson, Phys. Rev., 7
