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. It is a further advantage if the metals which fulfil the above conditions are cheap and durable. The exacting character of these requirements delayed the production of a reliable thermo-electric pyrometer until 1886, when Le Chatelier discovered that a junction formed of platinum as one metal, and an alloy of 90 per cent. of platinum and 10 per cent. of rhodium as the other, gave concordant results. In measuring the E.M.F. produced, Le Chatelier took advantage of the moving-coil galvanometer introduced by d’Arsonval, which possessed the advantages of an evenly-divided scale and a dead-beat action. This happy combination of a suitable junction with a simple and satisfactory indicator immediately established the reliability of the thermo-electric method of measuring temperatures. As platinum melts at 1755° C., and the rhodium alloy at a still higher temperature, a means was thus provided of controlling most of the industrial operations carried out in furnaces.