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J.F. Daniell

John Frederic Daniell was born in London, England, on 12 March 1790

His research in 1820 led to the invention of a dew-point hygrometer that measured relative humidity which afterwards became a standard instrument. His hygrometer was made with two thin glass bulbs that were hung from a base and joined with a glass tube. One of the glass bulbs held ether and a thermometer that collected and dissipated dew when the other bulb was slowly cooled and reheated. The condensing temperature was produced by evaporation of the ether. Daniell's hygrometer, as it was called, enabled the easy determination of vapor that existed in a given mass of atmosphere. The average temperature recorded by the device was the dew point. In 1823, he published Meteorological Essays which soon became a popular book.

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