The basis of thermocouples was established by Thomas Johann Seebeck in 1821 when he discovered that a conductor generates a voltage when it is subjected to a temperature gradient. Measuring this voltage requires the use of a second conductor material that generates a different voltage under the same temperature gradient. If the same material is used for the measurement, the voltage generated by the measuring conductor simply cancels that of the first conductor. The voltage difference generated by the two dissimilar materials can be measured and related to the corresponding temperature gradient.
Based on Seebeck’s principle, it is clear that thermocouples can only measure temperature differences and they need a known reference temperature to yield the absolute readings. The Seebeck effect describes the voltage or electromotive force (EMF) induced by the temperature gradient along the wire. The change in material EMF with respect to a change in temperature is called the Seebeck coefficient or thermoelectric sensitivity. This coefficient is usually a nonlinear function of temperature.