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.     The construction can be seen in the two examples below, with the metals being bismuth and antimony. The instrument at the right is by James W. Queen & Co. of Philadelphia; it was the top of the line model with 49 pairs of junctions, and cost $40.00 in the 1881 catalogue.

   The instrument is called a Differential Thermopile, which means that it is able to compare the thermal radiation of two different sources. The two examples below have horns pointing toward each of the two sources. The Transylvania instrument is by Elliott Brothers of London, and the one at Colby College is by Queen.
                               Transylvania University                                                          Colby College

   The thermopile at Union College was made by L.E. Knott of Boston, and cost about $15.00. The Washington and Jefferson and Washington and Lee differential thermopiles are 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. With an end-cap covering one set of thermo-junctions, the differential thermopile could be used to study, for example, the reflection of thermal radiation or the inverse-square law.
                     Union College                              Washington and Jefferson College               Washington and Lee University
 

At the left are two thermopiles in the Garland Collection of Classical Physics Apparatus at Vanderbilt University. The right-hand apparatus is marked "Ateliers Ruhmkorff / J. Carpentier / Ingr Constr / Paris. It was probably acquired about 1875.   The thermopile in the right-hand picture is in the collection at Dartmouth College, where it is described as a Line Thermo- pile. The aperture is clearly long and narrow, enabling it to be used with infra-red radiation emanating from a slit. The other side of the thermo-junction is held at room temperature inside the brass case. There is no maker's mark, but it looks like the Rhumkorff apparatus in the left-hand picture.

The Differential Thermoscope at the left was made by Max Kohl of Chemnitz, German. It has only one horn; the reference junctions on the opposite side face a sheet of brass at room temperature.     The cost of this device depended on the number of junctions. In the Kohl catalogue ca. 1900: 24 junctions -- 44 Marks 36 junctions -- 66 Marks 48 junctions -- 83 Marks 60 junctions -- 100 Marks (with the German Mark at about 25 cents)    This apparatus is in the Millington/Barnard Collection of the University of Mississippi.

The copper-constantan thermopile at the right was made by the Eppley Laboratory, a company best known for making Standard Cells for use with potentiometers. It is in the Greenslade Collection, and I would estimate that it dates from about 1930.    At the left is a tall (32 cm hight) differential thermopile whose elements are set into plaster of paris. It is unmarked, but came to the Greenslade Collection from Kirov in Russia.

This unmarked differential thermopile is in the apparatus collection at Holbart and William Smith Colleges in Geneva, New York.

The instrument at the left is listed in the 1916 Knott catalogue as "An Improved Mineralite Thermopile." This appears to be a Bismuth-Antimony thermopile with up to 120 pairs of junctions. The cost of the more sensitive of the two instruments in the catalogue was $14.50.    The catalogue gives data showing how the instrument was considerably more sensitive mthat a 20-pair thermopile in detecting the radiation from the sun on a "typical Boston Winter's day."    The apparatus is in the collection of Westminster College in New Wilmington, Pennsylvania.

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