In 1821 the Estonian physicist Thomas J. Seebeck made an accidental discovery: Not only does a potential (that is, voltage) difference exist between the two ends of an electrical conductor if these ends are at different temperatures, but also the voltage is a direct function of the temperature difference. If a circuit is made of a uniform material, the net loop voltage is zero. If, however, two different metallic conductors, such as platinum and palladium, are connected, a positive voltage is produced.
The thermocouple, after being calibrated using the melting points of certain pure substances such as lead (621.68°F/327.6°C), silver (1,762°F/961°C), and nickel (2,647°F/1,453°C), can measure temperatures nearly up to the melting points of its two components.
The Italian physicist Leopoldo Nobili (1784-1845) was slightly less ambitious and used a series of antimony bismuth bars to construct a thermopile in 1829. This was used to investigate infrared radiation and was connected to an astatic galvanometer that measured the voltage.
Later, the French industrial chemist Henry Le Chatelier was interested in measuring the temperatures greater than 930°F (500°C) commonly encountered in the cement industry. Using pure platinum and an alloy of platinum and rhodium in his thermocouple thermometer, he quickly realized that the purity and homogeneity of the two materials was of great importance.
By the 1890s, accurate, sturdy, and more reliable thermocouple thermometers were in widespread use in such places as steelworks. This enabled great strides to be made in the study of the effects of alloying elements in the properties of metals.
