Daniell hygrometer made by W. and S. Jones, c 1825.
© Science Museum / Science & Society Picture Library
John Frederic Daniell (1790-1845) was a Professor of Chemistry at King's College, London when he invented a new type of hygrometer - an instrument for measuring the degree of moisture in the air. In 1827 he published a description of a form of hygrometer consisting of a bent glass tube terminating in two bulbs, the one covered with muslin, the other of black glass. The closed glass system contains ether, which is condensed in the left-hand bulb which also contains a thermometer. Ether is poured over the muslin-covered bulb and the black ball, cooled by the evaporation of the ether within, is soon covered with dew. The dew-point temperature is a well-known function of the relative humidity of air. Daniell is best known for inventing the Daniell cell, an improvement over the voltaic cell used in the early history of battery development and devised in 1836.