An input sensor.
A thermocouple measures temperature. It works be having two dissimilar metals at the sensing end which creates a voltage that depends on the temperature. This voltage is normally tiny, (about 70 millionth of a volt per degree) so it is amplified before being used or stored with a datalogger.
Thermocouples cover a huge range of temperatures from liquid nitrogen up to molten lava. They are normally designed to cover a certain range and are identified by a "Type letter". For example a Type K thermocouple measures -200 C to +1350 C.
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Click on this link: Thermocouple