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The Bolometer


AN instrument a thousand times more sensitive to radiant heat than the thermopile, and capable of indicating a change of temperature as minute as 1-100,000th of a single Centigrade degree, deserves the attention of the physicist. When to these qualifications it can be added that the new instrument is far more prompt in its action, and more reliable than the thermopile for the quantitative measure of radiation, then, indeed, no apology is needed for a detailed description. The instrument is termed by its discoverer, Prof. S. P. Langley, the bolometer, or actinic balance. The earliest design of the inventor was to have two strips of thin metal, virtually forming arms of a Wheatstone's bridge, placed side by side in as nearly as possible identical conditions as to environment, one only of them being exposed to radiation. Such radiation would slightly warm the strip and therefore alter its electric resistance, and the amount of this change would be indicated by the movement of the needle of the galvanometer placed in the middle circuit of the “bridge.” For various reasons iron was eventually chosen as the material for the thin strips, as it combines the qualities of tenacity and laminability, with a greater sensitiveness in its electric resistance to temperature changes than either gold, platinum, or silver. Preliminary experiments made with a simple strip of iron in comparison with several delicate thermopiles showed the advantage of the new method of investigation. A large Elliott thermopile of sixty-three pairs, a very sensitive thermopile of sixteen small pairs, and a delicate linear thermopile of seven pairs of elements were selected. The iron strip taken was 7 millims. long,.177 millims. broad, and 0.004 millims thick. Its resistance was 0.9, ohm. The three former instruments were one after the other connected with a short-coil mirror galvanometer of sufficient delicacy. The same galvanometer was used in the bridge, the three resistances used with the strip being respectively.9,.4, and.4 ohm, and the total current employed being a little over half a weber. The result showed the sensitiveness of the three instruments and of the strip to heating by radiation to be respectively as 1, 4.1, 16.3, and 226.3.


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T., S. The Bolometer . Nature 25, 14–16 (1881).

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