. Instruments and methods used in radiometry, III - The photoelectric cell and other selective radiometers . ty 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
. Instruments and methods used in radiometry, III - The photoelectric cell and other selective radiometers . ty 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., 15, p. iii; , Dissertation, Bonn; 1906. This Bulletin. 14. p. i63; 1917. Coblentz] Selective Radiometers 517 to Ku Cm 70 ^o so ^o 30 - 20 - /O 1 • i 1 1 1 \ - 1 i - / c \ff / /■ - / \ \ B/ / ■ ,..Lr^ V- \ V, 1 - .90 MS ,50 .55 GO ,65 .70 ,7^ Fig. 3.—Distribution of energy of a gas-filled tungsten lamp as observed with:A, photoelectric cell; and B, with a thermopile. Curve C is the true sensi-tivity of the photoelectric cell for an equal energy spectrum. 7g^ Fig. 4.—Photoelectric sensitivity: A=calcium; B=rubidium. Sensibility of the average eye: C110990°—19 3 5i8 Bulletin of the Bureau of Standards ivd. 14 trsry to published statements,^^ there is no marked similaritybetween the photoelectric sensitivity curves of these metals andthe wave-length sensibility curve of the eye. (/) TIic Auxiliary Gahometer.—At best a sensitive electrometeris slow to act, and it is difficult to operate, on account of leakage,etc. Attention has already been called ^^ to the usefulness of ahigh-resistance ironclad Thomson galvanometer to measurephotoelectric currents. There is, of course, nothing new in theuse of a high-resistance galvanometer in a circuit of this typewhich has a ver>^ high external resistance. However, an exam-ination of the older data -^ concerning high-resistance galva-nometers (up to 300 000 ohms) shows that the sensit
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