thermocouples are a widely used type of temperature sensor. They are cheap, interchangeable, have standard connectors and can measure a wide range of temperatures. The main limitation is accuracy; system errors of less than 1 °C can be difficult to achieve.
Principle of operation
In 1822, an Estonian physicist named Thomas Johann Seeback discovered that when any conductor (such as a metal) is subjected to a thermal gradient, it will generate a small voltage. Thermocouples make use of this so-called Peltier-Seeback effect.
Thermocouples produce an output voltage which depends on the temperature difference between the junctions of two dissimilar metal wires. It is important to appreciate that thermocouples measure the temperature difference between two points, not absolute temperature.
In most applications, one of the junctions — the "cold junction" — is maintained at a known (reference) temperature, whilst the other end is attached to a probe. For example, in the image below, the cold junction will be at copper tracks on the circuit board. Another temperature sensor will measure the temperature at this point, so that the temperature at the probe tip can be calculated.

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