In early 1821, Thomas Seebeck searched experimentally for a relation between electricity and heat. He joined two wires of two dissimilar metals to form a circuit. He discovered that if one junction is heated to a high temperature, and the other junction remained at a cooler temperature then the galvanometre connected at their ends shows a deflection. This is known as Seebeck Effect. The e.m.f. generated in the circuit is called thermoelectric e.m.f. The resulting current is known as thermoelectric current.The two junction circuit is called a thermocouple.
In this process heat energy is directly converted into electrical energy. The wire pairs can be composed of noble metals (such as Platinum, iridium , silver,osomium,gold and rhdom) or base metal ,(such as copper,iron or nickal- copper alloy ,lead and zinc ) Thus it remains true for any pair of metals.
Thermocouples are the most widely used temperature sensors in industry due to their low cost, simplicity, size and useable temperature range. The electromotive force is a function of the temperature gradient.The thermo- e.m.f. produced is very small, of the order of mV per every degree of temperature difference.
The Seebeck effect is reversible, i.e., if the hot and cold junctions are interchanged, the direction of e.m.f. (and hence current) reverses.The greater the separation of the metals forming the thermocouple in the series, greater is the thermo e.m.f. produced.
The thermo e.m.f. of many thermocouples has been measured as a function of the temperature T of the hot junction, when the cold junction is maintained at 0°C. Its temperature dependence is given by
Ē= aT + 1.5BT
Where a and B are constants (called thermoelectric coefficients) which depend on the nature of the metals.
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