Pyrometry: A Practical Treatise on the Measurement of High Temperatures. Charles R. Darling

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СКАЧАТЬ ” ” ” 1063 1945 Copper (in reducing atmosphere) ” ” ” 1083 1982

      For higher temperatures the melting points of nickel (1452° C.) and palladium (1549° C.) are employed, but the accuracy in these cases is not so certain as with the substances named in the table. A useful point, intermediate between copper and nickel, has been established by E. Griffiths, and is obtained by heating nickel with an excess of graphite, when a well-defined eutectic is formed which freezes at 1330° C., or 2426° F.

      Temperatures above the Present Limit of the Gas Thermometer.—As it is not yet possible to compare an instrument directly with the gas thermometer above 1550° C., all higher temperatures must be arrived at by a process of extrapolation. By careful observation of a physical change at temperatures up to the limit of 1550° C., the law governing such change may be discovered; and assuming the law to hold indefinitely, higher temperatures may be deduced by calculation. An amount of uncertainty always attaches to this procedure, and in the past some ludicrous figures have been given as the result of indefinite extrapolation. Wedgwood, for example, by assuming the uniform contraction of clay, gave 12001° C., or 21637° F., as the melting point of wrought iron, whereas the correct figure is 1520° C., according to the gas scale. Even in recent times, the extrapolation of the law connecting the temperature of a thermal junction with the electromotive force developed, obtained by comparison with the gas scale up to 1100° C., led Harker to the conclusion that the melting point of platinum was 1710° C., a figure 45 degrees lower than that now accepted. The laws governing the radiation of energy at different temperatures, however, appear to be capable of mathematical proof from thermodynamic principles, and temperatures derived from these laws are in reality expressed on the absolute or thermodynamic scale. Extrapolation of these laws, when used to deduce temperatures by means of radiation pyrometers, appears to be justified; but it is still desirable to extend the gas scale as far as possible to check such instruments. Assuming the radiation laws to hold, it is possible to determine the highest temperatures procurable, such as that of the electric arc, with a reasonable degree of certainty.

[1] For a fuller account of the thermodynamic scale, see the author’s treatise Heat for Engineers, pp. 391–2.

      

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