Thermocouple

[1] An electrical temperature sensor. As opposed to a resistance thermometer, the thermocouple operates without an external voltage; it consists basically of two conductors of dissimilar metals, typically copper/constantan, iron/ constantan, nickel-chromium/nickel, which are connected at their ends; one joint, the hot junction, is exposed to the temperature to be measured, the other joint, the cold or reference junction, is held at a fixed, known temperature; a thermoelectrical voltage is developed between the two junctions which is proportional to the temperature difference between the junctions
[2] Device which generates electricity, using principle that if two unlike metals are welded together and junction is heated, a voltage will develop across the open ends.