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When two wires made of different metals are connected to make a loop, and the two junctions between the wires are held at different temperatures, a voltage is produced and electrical current can be detected with a sensitive current meter. This is the Seebeck Effect, discovered by Thomas Seebeck in 1821. The effect is multiplied when there are more than two junctions in series in the circuit, with alternate junctions in close thermal contact with each other. By measuring the current, one can measure the temperature difference of the junctions. The Differential Thermopile was invented by Macedonio Melloni (1798-1854), an Italian physicist who worked in France and Italy. Melloni's research dealt with thermal radiation, and he developed the thermopile to make quantitative measurements of the intensity of the radiation. Washington and Lee’s differential thermopile was made by E.S. Ritchie. The model with 20 pairs of junctions sold for $25.00 in the 1881 catalogue; for $40.00 you could get the 49 pair model.