In early 1821, Thomas Seebeck searched experimentally for a relationship between electricity and heat by joining two wires of dissimilar metals (copper wire and bismuth wire) to form a loop or circuit. Seebeck discovered that two dissimilar metal wires, joined at both ends, would produce a current in the thermoelectric circuit when the junctions (soldered ends) are at different temperatures. The current produced is a function of the junction temperature and the type of metals used. This relationship between thermal EMF (Electromotive Force or Voltage) and temperature is known as the Seebeck effect. All dissimilar metals exhibit the Seebeck effect, but the voltage/temperature relationship varies depending on the metals. Various combinations of metals are used depending on the output voltage required and the temperature range. This principle is the basis for the thermocouple.