. Instruments and methods used in radiometry . tive radiometer (deflection = 50 cm per mm2). The driftingof the zero is a far less serious matter than in a bolometer or athermopile, and is not troublesome in less sensitive instruments. The dimensions to be chosen for the vane will depend on itsdistance from the spectrometer slit. The image of the slit canbe made to just cover the vane by placing a short focus (say 10 to12 cm) condensing mirror between the spectrometer slit and theradiometer vane. Since mica vanes are not opaque to all rays, andsince the lampblack may rub off, it is better to u
. Instruments and methods used in radiometry . tive radiometer (deflection = 50 cm per mm2). The driftingof the zero is a far less serious matter than in a bolometer or athermopile, and is not troublesome in less sensitive instruments. The dimensions to be chosen for the vane will depend on itsdistance from the spectrometer slit. The image of the slit canbe made to just cover the vane by placing a short focus (say 10 to12 cm) condensing mirror between the spectrometer slit and theradiometer vane. Since mica vanes are not opaque to all rays, andsince the lampblack may rub off, it is better to use platinumvanes, about mm thick, blackened electrolytically and thensmoked. In addition to having the radiometer enclosed in a heavy metalcase to shield it from temperature changes, it is advisable tocover the instrument with a metal cylinder and to pack woolbetween it and the inner case. In warm weather it has beenfound that the vanes become easily electrified from the mercury 414 Bulletin of the Bureau of Standards. \voi. 4, m I. jm. u w I cbm I
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