A hygrometer is an instrument used in meteorological science to measure the humidity or amount of water vapor in the air. Several major types of hygrometers are used to measure humidity. Mechanical hygrometers make use of the principle that organic substances (particularly finer substances such as goldbeater’s skin [ox gut] and human hair) contract and expand in response to the humidity. Contraction and expansion of the hair element in a mechanical hygrometer causes the spring to move the needle on the dial.

Source: Britannica