SM215     Thermocouple      circa 1940     Heat

  In 1821, Seebeck found that by heating one of the junctions of a bimetallic circuit, an electric current is produced.   Metals form a thermo-electric series by producing different coefficients of emf with temperature with respect to lead as a reference.   For example, Bismuth is +97μV with Nickel +22μV and tin and Copper being  –1μV.   The coefficient of Silver is  -2.5μV and that of Antimony is  -25μV.   The best combination of materials found empirically for producing the highest emf is Bismuth and Antimony and the combination is used in the thermopile, which consists of a large number of these junctions in series.

  In an attempt to produce electrical energy directly from heat, a rod of bismuth is produced in a twice-bent form and copper is used as the other metal forming the junctions.   This assembly can be seen as SM215.   If one of the junctions is kept in ice water and the other in boiling water, an emf of 0.01 Volts is produced and the current can be large since there is a low electrical resistance.   Several of these can be connected in series to form a thermo-electric battery.   The conversion efficiency of such a battery is found to be very low (about 1%).