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Lippmann electrometer. 19th-century illustration of the capillary electrometer developed in 1873 by Franco-Luxembourgish physicist Gabriel Lippmann (1845-1921). An electrometer is used to accurately measure electric charge or voltage. This design consists of a capillary tube half-filled with mercury with sulphuric acid above it. Metal wires connect the mercury and the acid. An electric current changes the surface tension of the mercury, and the distance it moves up the capillary tube is a measure of the strength of the voltage. This illustration is from 'Physique Populaire' (Emile Desbeaux, 1891).(KEYSTONE/SCIENCE PHOTO LIBRARY SPL/SCIENCE PHOTO LIBRARY)