When two dissimilar metals such as iron and copper are joined at both ends to form a closed circuit, and one of the junctions is at a higher temperature than the other (Fig. 128), a current is set up. The e.m.f. driving this current is called a 'thermoelectric e.m.f.', and the phenomenon is known as the thermoelectric effect or Seebeck effect after the German physicist who discovered it in 1821.
Usually a thermoelectric e.m.f. is very small, only a few millionths of a volt. For a copper-iron circuit it is found to be about 7lV for every degree Centigrade of temperature difference between the junctions; for antimony and bismuth it is as high as 100 lV per deg C, while for copper and Constantan (55 per cent copper, 45 per cent nickel), the two metals most often used in practice, it is 40 lV per deg C of temperature difference.
A pair of dissimilar metals welded together at their junction forms what is called a thermocouple. By arranging several thermocouples in series, as shown in Fig. 129, the e.m.f.s add together to give an appreciable output; this arrangement is known as a thermopile.
Although thermopiles have been constructed to deliver e.m.f.s of a few volts, the thermoelectric effect is rarely used at present as a source of energy. Its main application lies in the measurement of temperature. A form of hotwire ammeter (p. 135) for measuring alternating currents incorporates a thermocouple whose junction is heated indirectly by the current being measured. The thermal e.m.f. in the junction sets up a direct current which is measured by a moving-coil galvanometer.
The reverse of the Seebeck effect, called the Peltier effect, occurs when a current is passed through a circuit of two dissimilar metals: heat is produced at one of the junctions and absorbed at the other. The latter junction is therefore cooled, and this cooling effect has been used as the basis of a novel type of refrigerator.
* For a simple explanation of the thermoelectric effect we must accept that when two dissimilar metals are pressed together free electrons drift haphazardly across the junction. Because of the different atomic structure of each metal, electrons pass more readily across the boundary in one direction than in the other. This results in a displacement of charges, making one metal positive and leaving the other negative.
The voltage between the metals is called a contact potential difference, and is influenced by the temperature of the boundary. By keeping one junction at a higher temperature than the other, the unequal drift of electrons past each junction maintains a steady potential difference, and a thermoelectric e.m.f. can be obtained.
(From ‘Electricity Made Simple’ by Leslie Basford, ISBN 0 7506 0385 2)
[There is NO ELECTROLYSIS occurring here]
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