Alluard Hygrometer

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Emile Alluard, professor of physics at the University of Clermont-Ferand and director of the meteorological observatory on the nearby Puy-de-Dôme, described this type of hygrometer in 1878. A modification of Regnault's instrument, it consists of a square vessel made of polished nickel-plated brass. At either side of the vessel, but not touching it, are strips of similar material. In use, the vessel would be filled with ether, and this ether would be cooled by evaporation by means of an aspirating bulb. When dew appears on the shiny surface of the vessel, a thermometer in the vessel indicates the temperature of the ether at that point. A second thermometer measures the ambient temperature.
Robert A. Millikan described the Alluard hygrometer as being one of the "most perfect forms of the dew-point hygrometer" in his Mechanics, Molecular Physics and Heat, a popular college text that was first published in 1903 and that aimed "to present Physics as a science of exact measurement." This example is marked "A Sign of Quality WELCH A Mark of Service / W. M. Welch Scientific Company / ESTABLISHED 1880 / 1515 SEDGWICK ST. CHICAGO, U.S.A." It was made between 1921 (when the W. M. Welch Scientific Company began as such) and 1960 (when it became Welch Scientific). It is missing the aspirating bulb and both thermometers.
Ref: M. Allouard, "Nouvel Hygromètre a Condensation," Journal de Physique et le Radium 7 (1878): 328-330.
W. M. Welch Scientific Co., Catalog G (Chicago, 1935), p. 157.
Currently not on view
W. M. Welch Scientific Company
place made
United States: Illinois, Chicago
overall: 9 in x 4 1/2 in; 22.86 cm x 11.43 cm
ID Number
catalog number
accession number
Credit Line
U.S. Weather Bureau
See more items in
Medicine and Science: Physical Sciences
Measuring & Mapping
Thermometers and Hygrometers
Data Source
National Museum of American History


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