The production of a voltage between dissimilar metals with their ends at different temperatures is a function of the three thermoelectric effects in metals:

  • The Seebeck effect: Thomas Johann Seebeck (1770-1831) was the first person to demonstrate the thermoelectric effects of producing currents which occur in a circuit comprising two metals where the two welds are at different temperatures. He did not fail to note the phenomenon caused by a temperature difference along a homogeneous conductor; a phenomenon which would be rediscovered thirty years later by William Thomson and called the Thomson Effect.
  • The Peltier effect: Jean Charles Athanase Peltier, French physicist (1785-1845), gave up the profession of watchmaker at the age of thirty to devote himself to experimental physics; he is known for his discovery in 1834 of the Peltier effect: when an electric current passes through the junction of two dissimilar metal conductors, there is a rise or fall in temperature depending on the direction of the current; the amount of heat released or absorbed is proportional to the strength of the current. It is in a sense the converse of the Seebeck effect. Passing a current can therefore cause heat to be absorbed; this effect is used in some small refrigerators or to cool electric circuits.
  • The Thomson effect: This was discovered by Lord Kelvin (then Sir William Thomson) – but already noted by Seebeck – who proved its existence by experiment after demonstrating it theoretically. It is a sort of Peltier effect but between contiguous sections of the same metal bar. The Thomson effect relates to the production – or absorption – of heat caused by passing a current through a section of a conductor where there is a difference in temperature between the ends of the section.