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

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.