Seebeck, Thomas Johann (1770–1831)

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SEEBECK, THOMAS JOHANN (1770–1831)

Thomas Johann Seebeck, German physician, physicist, and chemist, was born April 9, 1770, in the Hanseatic town of Revel (now Tallinn Estonia). Seebeck studied medicine in Berlin and Goettingen, where in 1802 he took a physician's doctor degree. He lived in Jena (Thuringia) between 1802 and 1810 as an independent scholar not practicing medicine. In the Jena period Seebeck met poet Johann Wolfgang Goethe, who carried on his own studies on chromatics. From 1806 untill 1818 Seebeck consulted and informed Goethe about his optical observations. He discovered entoptical colors in 1813. These are color figures which originate inside glass volume after cooling. The phenomenon of entoptical colors was broadly documented by Goethe in his treatises on theory of colors. Seebeck became a member of the Academy of Sciences in Berlin in 1814. After stays in Bayreuth and Nuernberg (Bavaria) he moved to Berlin in 1818. Goethe hoped that Seebeck, in the prestigious position of an academician, would help to lend respectability to his theory of colors. Instead, the contact between the two men loosened in the 1820s, as Seebeck seems to have distanced himself cautiously from Goethe's theory. In Berlin Seebeck devoted most of his time to experiments measuring the influence of heat on magnetism and electricity. Seebeck died December 10, 1831, in Berlin.

Seebeck's outstanding scientific achievement was the discovery of one of the three classical thermoelectric effects, which are the Seebeck, the Peltier, and the Thomson effects. Seebeck's discovery was the first, dating from 1822–1823, followed by that of Jean-Charles-Athanase Peltier in 1832 and that of William Thomson in 1854. Seebeck observed that an electric current in a closed circuit comprised different metallic components if he heated the junctions of the components to different temperatures. He noted that the effect increases linearly with the applied temperature difference and that it crucially depends on the choice of materials. Seebeck tested most of the available metallic materials for thermoelectricity. His studies were further systematized by the French physicist Antoine-Cesar Becquerel (1788–1878). It has become common after Becquerel to classify materials according to their so-called thermoelectric power. The difference in thermoelectric power of the applied materials determines the strength of thermoelectricity. Since the Seebeck effect can only be observed if two dissimilar conductors are employed, one obtains therewith only access to differences of values of thermoelectric power. Absolute values can be ascertained through the Thomson effect, which states that in a single homogeneous material heat develops (or is absorbed) in case electric current flows parallel (or antiparallel) to a temperature gradient. The strength of this effect is expressed by the Thomson coefficient, which is a temperature-dependent quantity varying from material to material. The absolute value of the thermoelectric power of a material is given by the temperature integral of the Thomson coefficient multiplied by the inverse temperature.

Seebeck understood that his effect might be used for precision measurements of temperature differences, and indeed it is exploited for this purpose in modern thermoelements. The basic device in these thermoelements is an electric circuit comprising two metallic components of different thermoelectric power, a thermocouple. The temperature difference to which the thermocouple is exposed is measured through the voltage built up in the circuit, according to Seebeck. Thermoelements with various combinations of materials for their thermocouples are used in the temperature range from –200°C to 3,000°C. their simplicity and robustness makethem an almost universal element in the fields of temperature recording and temperature regulation. In addition to their higher precision as compared to conventional thermometers, these thermoelements have the additional advantage that they can be introduced into apertures fitting to the thinnest technologically realizable wires. Thermoelements also have a much smaller heat capacity than conventional thermometers (e.g. mercury thermometers) and therefore interfere much less with the body to be measured.

Another application of the Seebeck effect is to be found in detectors of small quantities of heat radiation. These sensitive detectors comprise a thermopile, a pile of thermocouples (small pieces of two different metals connected in V form and put into series). Half of the junctions of the thermopile are shielded within the detector, whereas the other half are exposed to external heat radiation which is recorded through a voltmeter in the thermopile circuit.

Barabara Flume-Gorczyca