In 1821, the German physicist Thomas Johann Seebeck discovered that a magnetic needle held near a circuit made up of two dissimilar metals got deflected when one of the dissimilar metal junctions was heated. At the time, Seebeck referred to this consequence as thermo-magnetism. The magnetic field he observed was later shown to be due to thermo-electric current. In practical use, the voltage generated at a single junction of two different types of wire is what is of interest as this can be used to measure temperature at very high and low temperatures. The magnitude of the voltage depends on the types of wire being used. Generally, the voltage is in the microvolt range and care must be taken to obtain a usable measurement. Although very little current flows, power can be generated by a single thermocouple junction. Power generation using multiple thermocouples, as in a thermopile, is common.
The standard configuration for thermocouple usage is shown in the figure.
Briefly, the desired temperature T is obtained using three inputs—the characteristic function E(T) of the thermocouple, the measured voltage V, and the reference junctions temperature T'.
The solution to the equation E(T) = V + E(T) yields T.
These details are often hidden from the user since the reference junction block (with T thermometer), voltmeter, and equation solver are combined into a single product.