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Thomson quadrant electrometer. 19th-century illustration of the quadrant electrometer developed in 1867 by British physicist William Thomson (1824-1907, later Lord Kelvin). An electrometer is used to accurately measure electric charge or voltage. This extremely sensitive design has an aluminium disc (suspended at centre on a torsion fibre) cut into four segments. The insulated segments are connected diagonally in pairs. When charged, one aluminium segment is attracted to one pair of segments and repelled from the other segment. This deflection (a measure of the size of the electric charge) is measured by a beam of light reflected from a mirror attached to the charged segment. This illustration is from 'Physique Populaire' (Emile Desbeaux, 1891).
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