In 1821 Thomas Seebeck discovered the physical phenomenon that when you join two dissimilar metals on both ends and heat one end, a continuous current is created in the thermoelectric circuit. If the other end is broken, a voltage can be measured that is proportional to the temperature difference between the heated junction (measuring or “hot” junction) and the open end (reference or “cold” junction) at which the measurement is taken. This physical property, called the Seebeck effect, is the basis of the thermocouple sensor.