=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. CHAPTER III THERMO-ELECTRIC PYROMETERS =General Principles.=—Seebeck, in 1822, made the discovery that when a junction of two dissimilar metals is heated an electromotive force is set up at the junction, which gives rise to a current of electricity when the heated junction forms part of a closed circuit. Becquerel, in 1826, attempted to apply this discovery to the measurement of high temperatures, it having been observed that in general the E.M.F. increased as the temperature of the junction was raised. No concordant results were obtained, and the same fate befell the investigations of others who subsequently attempted to produce pyrometers based on the Seebeck effect. These failures were due to several causes, but chiefly to the non-existence of reliable galvanometers, such as we now possess. It was not until 1886 that the problem was satisfactorily solved by Le Chatelier of Paris.