Thermocouples are two wires of dissimilar metals, joined at one end. Changes in the temperature at the "hot junction" (measuring end) induce a change in electromotive force (emf) at the "cold junction", where it can be input into a controller or indicator. As the temperature increases, this output emf also rises, though not completely linearly. Thermocouples can measure temperatures over wide ranges, fairly inexpensively. They are very rugged, but they are somewhat less accurate vs. RTDs and thermistors. United Electric Controls (UE) offers type J, K, E and T Thermocouple Terminal Head Styles as well as Beaded Thermocouple types: K, R, S & B.