| 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.
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.
| 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-
|| 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
(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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