M. Allouard, director of Puy-de-Dôme Observatory in France, 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 "CENTRAL SCIENTIFIC CO. / CHICAGO CENCO U.S.A. / LABORATORY APPARATUS."
Ref: M. Allouard, "Nouvel Hygromètre a Condensation," Journal de Physique et le Radium 7 (1878): 328-330. .
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