. Instruments and methods used in radiometry - II . mm diameter andabout mm thick. The disksoverlapped so that the 20 junc-tions occupied a space 20 mmlong. The actual area exposedwas therefore greater than in theBi-Ag pile, which had a receivingsurface of about 16 mm2. Thiscomparison shows that the radia-tion sensitivity of the bismuth-silver is about times that ofthe ordinary Rubens we consider that in spectro-radiometry we are interested inthe length of the spectral linewhich can be utilized, we shouldhavemadethelengthoftheBi-Agpile2omm, radiatio
. Instruments and methods used in radiometry - II . mm diameter andabout mm thick. The disksoverlapped so that the 20 junc-tions occupied a space 20 mmlong. The actual area exposedwas therefore greater than in theBi-Ag pile, which had a receivingsurface of about 16 mm2. Thiscomparison shows that the radia-tion sensitivity of the bismuth-silver is about times that ofthe ordinary Rubens we consider that in spectro-radiometry we are interested inthe length of the spectral linewhich can be utilized, we shouldhavemadethelengthoftheBi-Agpile2omm, radiation sensitivity of the Bi-Ag pile would therefore beincreased in the ratio of 20 to 12, which would give a total deflec-tion of about 100 cm instead of 60 cm as observed. It is there-fore safe to say that, comparing equal lengths of receiving surfaceutilized on a spectral line, the radiation sensitivity of this type ofbismuth-silver pile is 4 times that of the old type of low resistance,iron-constantan pile. If we compare the bismuth-silver with the. -Ag THERMOPILEFig. 3 20 Bulletin of the Bureau oj Standards [VcUq fin p. wire type - of iron-const ant an pile, which has a resistance to ohms and a radiation sensitivity times that of theold type of Rubens pile, it is a fair estimate to say that this bismuth-silver pile has a radiation sensitivity at least 3 times as great as thenew type of iron-constantan thermopile. One important point usually not considered is the heat capacityand hence the speed of attaining temperature equilibrium in theseinstruments. In the present radiation sensitivity tests the halfperiod of the galvanometer was 2 seconds. When joined with thebismuth-silver pile the half period was lengthened to about , which is practically the same as with the new type of fine-wire iron-constantan pile described in a previous This isnot an excessively long period and the galvanometer mirror re-turns quickly to its zero position. With the old
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