Thermocouple probes operate on the principle that an electromagnetic force (emf) is generated when heat is applied to the junction of two dissimilar metals (sending junctions). At the other end of the wires, usually as part of the input instrument, is another junction called the reference junction. The temperature is inferred based on the emf difference between the sensing and the reference junction, which is at a known temperature. Thermocouple probe junction types include exposed junction, ungrounded junction, and grounded junction. Thermocouples offer the greatest temperature range and are more cost effective but are the least sensitive and accurate of the sensor types. A thermocouple probe is suitable for use from -200 to 2000 degrees C.