[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.
Although any heated junction of metals will give rise to an electromotive force, it does not follow that any pair, taken at random, will be suited to the purposes of a pyrometer. A junction of iron and copper, for example, gives rise to an E.M.F. which increases with the temperature up to a certain point, beyond which the E.M.F. falls off although the temperature rises, and finally reverses in direction—a phenomenon to which the name of “thermo-electric inversion” has been applied. Evidently, it would be impossible to measure temperatures in this case from observations of the electromotive force produced, and any couple chosen must be free from this deterrent property. Moreover, the metals used must not undergo deterioration, or alteration in thermo-electric properties, when subjected for a prolonged period to the temperature it is desired to measure. These and other considerations greatly restrict the choice of a suitable pair of metals, which, to give satisfaction, should conform to the following conditions:—
1. The E.M.F. developed by the junction should increase uniformly as the temperature rises.
2. The melting point of either component should be well above the highest temperature to be measured. An exception to this rule occurs when the E.M.F. of fused materials is employed.
3. The thermo-electric value of the couple should not be altered by prolonged heating.
4. The metals should be capable of being drawn into homogeneous wires, so that a junction, wherever formed, may always give rise to the same E.M.F. under given conditions.