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Seebeck discovered that if two wires made from dissimilar metals are connected at both ends to make two junctions, when one end is heated, a small amount of current would flow through the circuit ( see Fig. 1 below). In Fig. 1a notice both ends of wires A and B are twisted together and one end is heated. Fig. 1b shows that if the circuit is altered so that the two wires are twisted together to make a junction at only one end and this junction is heated, the two wires will now produce an open-circuit voltage that is called the Seebeck voltage. The open-circuit voltage is proportional to the amount of heat added to the junction.
Above: Fig. 1 (a) The Seebeck effect shown by connecting
two dissimilar metals at both ends to make a thermocouple junction. The
Seebeck current will flow in this closed circuit. (b) Two dissimilar metals
are joined at one end to make a thermocouple junction to produce a Seebeck
voltage.
If a voltmeter is used to read the millivoltage that the thermocouple produced, a second junction of dissimilar metals is produced at the point where the thermocouple wire is connected to the copper wires of the voltmeter. This junction will produce a small amount of voltage in opposition to the original voltage produced at the thermocouple junction. One way to get around this problem is to extend the copper wire from the meter to point where a second junction can be produced in such a way that the voltage from this junction can be controlled.
