Hygrometer

A hygrometer is an instrument that measures humidity, the amount of water vapor present in the air, historically by measuring related properties such as the expansion of a moisture-sensitive material. Leonardo da Vinci built a prototype hygrometer around 1480, and the Italian physician Francesco Folli and the English scientist Robert Hooke made practical improvements to humidity-measuring instruments in the 17th century. The Swiss scientist Horace Benedict de Saussure invented the hair-tension hygrometer in 1783, using the expansion and contraction of a strand of human hair as its sensing element, a design that dominated humidity measurement for over a century.