Seebeck Effect
It states that, “when two dissimilar metal wires are joined to form a complete electric circuit and the two junctions are maintained at different temperatures, an electromotive force (emf) is set up to establish flow of current”. Magnitude of emf generated is directly proportional to magnitude of temperature difference of two junctions and materials used for wires or conductors. Let us consider two wires made up of different materials. Ends of both wires are joined to form two contact points or junctions.
Net Emf Generated
Following emfs will be generated due to Thomson effect and Peltier effect.
Thomson effect
- According to Thomson effect, if one end of wire is heated, then a Thomson emf is generated or developed between two ends of that single wire.
- Two Thomson emfs will be generated, since there are two wires.
Peltier effect
According to Peltier effect, when a junction formed due to contact of ends of two wires made-up of dissimilar metals is heated or cooled, a Peltier emf is generated. Two peltier emfs will be generated, since there are two junctions. Therefore, Net emf will be resultant of two peltier emfs and two Thomson emfs. However, in commercial instruments, materials for wires or conductors are so selected that, emfs due to Thomson effect will be negligible and the instruments will measure Peltier emfs only. It is important to note that, magnitude of emf generated by Peltier effect is proportional to temperature of hot junction. If temperature at one junction (reference junction, i.e. cold junction) is kept constant, then effective emf will be generated due to change in temperature at another junction, i.e. hot junction. Temperature difference between hot and cold junction can be measured by this effective emf generated in thermocouple.
Therefore, hot junction is called as measuring end, where temperature is to be measured and cold junction is called as reference junction, where reading of temperature difference between two junctions is taken.