Hygrometer

A hygrometer is an instrument used to measure the humidity of the air, that is, the amount of water vapor present, using methods that have included the expansion and contraction of organic materials such as hair or paper, the difference in temperature between wet and dry thermometer bulbs, and modern electronic sensors that measure humidity through changes in electrical capacitance or resistance. Early hygrometers were devised from the fifteenth century onward, with Leonardo da Vinci among the first to describe a device using absorbent material to detect changing air moisture, and the instrument was substantially refined in the eighteenth century, notably by the Swiss scientist Horace-Benedict de Saussure, who developed a widely adopted hair hygrometer in 1783 exploiting human hair's tendency to lengthen as humidity increases. Hygrometers remain essential instruments in meteorology and climate monitoring, and are also widely used in agriculture, museum and archive conservation, industrial process control and building management, wherever maintaining or measuring a specific humidity level matters.