Thermocouples can measure temperatures over wide ranges, inexpensively. They are very rugged, but they are not as accurate as RTDs and thermistors.
A thermocouple is basically two strips of wires of dissimilar metals. These metal wires are joined at one end and the voltage is measured at the other end. Changes in the temperature at the juncture induce a change in electromotive force (emf) at the other end. As the temperature goes up, this output emf of the thermocouple rises, though not necessarily linearly.
