File:Instruments and methods used in radiometry - II (1913) (14757195225).jpg

Identifier: instrum97631913188188unse (find matches)
Title: Instruments and methods used in radiometry - II
Year: 1913 (1910s)
Authors:
Subjects: measurements and use of instruments thermometry, pyrometry and calorimetry Heat flow, thermal and thermodynamic processes
Publisher: National Bureau of Standards
Contributing Library: NIST Research Library
Digitizing Sponsor: NIST Research Library

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lead^ wires are illustrated in position in Fig. 10. The bolometerstrips are 10 by 0.6 mm and have a computed thickness of 0.0003mm and resistances of 5.604 ohms (exposed strip) and 5.664 ohms,respectively. The resistance of the complete bolometer is about8.9 ohms (depending, of course, upon the battery current and thevacuum) with a temperature coefficient of 0.0230 ohms per degree.The resistance of the galvanometer is 5.09 ohms at 200 C, with atemperature coefficient of 0.0194 ohms per degree. The rear side of each bolometer strip is left bright, and the frontside is painted with a mixture of equal parts of lampblack and ofplatinum black, which is made into a smooth, thick, paste withturpentine, or with a dilute alcoholic solution of shellac. This isapplied with a brush made of a thin strand of a fine silk thread.By starting at one support and by making one uniform strokealong the bolometer strip, and not stopping until after arriving at 40 Bulletin of the Bureau of Standard: :•- 3
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Cobientz) Instruments and Methods of Radiometry 41 the other end, there is no danger of tearing the strip. After this,the bolometer is blackened a little more by smoking over a candleflame. Chemically precipitated platinum black should be usedinstead of the powdered electrolytic material because it is blacker.However, a layer of platinum black, if successfully depositedelectrolytically upon a bolometer strip, is the best of all. The degree of evacuation is measured by means of the length ofthe cathode dark space in a small vacuum tube having disk elec-trodes of aluminum. There is practically no leakage when thestopcocks are in good condition and the vapor pressure from thecement is a fairly constant value, so that a cathode dark space of2 mm (white C02 glow of hydrocarbon vapors) is easily maintainedfor weeks. The radiation sensitivity is lower for hydrocarbonvapors than for nitrogen or hydrogen, so that in practice air isadmitted and the hydrocarbon vapors are pumped out. Forexample,

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