. Instruments and methods used in radiometry, III - The photoelectric cell and other selective radiometers . ^ distribution as obsen^ed, curve B,with a thermopile which is a nonselective radiometer. Anironclad Thomson galvanometer was used with both of theseinstruments. From these cun-es it will be noticed that the photoelectric cellis not sensitive to the deep red. In the blue it is very sensitive, sothat as a radiometer in this region of the spectrum it siupassesthe thermopile or bolometer in sensitivity. Ciu^e C gives thetrue sensitivity of the photoelectric cell of potassium hydrideresulti
. Instruments and methods used in radiometry, III - The photoelectric cell and other selective radiometers . ^ distribution as obsen^ed, curve B,with a thermopile which is a nonselective radiometer. Anironclad Thomson galvanometer was used with both of theseinstruments. From these cun-es it will be noticed that the photoelectric cellis not sensitive to the deep red. In the blue it is very sensitive, sothat as a radiometer in this region of the spectrum it siupassesthe thermopile or bolometer in sensitivity. Ciu^e C gives thetrue sensitivity of the photoelectric cell of potassium hydrideresulting from stimulating the cell with equal energies throughoutthe spectrum. It is the ratio of curv^e .4 to curv^e B, in view of Vo/TsZ50 200. ISO 100 Fig. 2.—Yariaiion in radiation sensitivity of a photoelectric cell with change in applied voltage the fact that for this range of intensities the photoelectric ciurentis proportional to the energy stimulus; or it is the wave-lengthsensitivity cuTv-e. As already mentioned, the maximum sensitivity shifts towardthe short wave lengths w4th decrease in atomic w^eight of thealkali metal. The data published by Ives ^^ show a great varia-tion in the wave-length sensitivity cur\^es of potassium Fig. 4, cur\e A is shown the wave length, photoelectric sensi-tivity of calcium -^ and cun^e B of rubidium, which has itsmaximum sensitivity at The visibility curve of theaverage eye ^^ (125 observers) is illustrated in cur\^e C. Con- 2 Ives, Astrophys. Jour., 40, p. 1S2; 1914. In a recent paper (Astroph\-s. Jour., 46, p. 241, 1917) Ivesshows that the spectral sensitivity changes with time, becoming relatively more sensitive in the Pohland Prinssheim, Verh. Phys. Gesell
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