A hygrometer is an instrument used to measure the moisture content or the humidity of air or any gas. The best known type of hygrometer is the "dry and wet-bulb psychrometer", best described as two mercury thermometers, one with a wetted base, one with a dry base. The water from the wet base evaporates and absorbs  heat causing the thermometer reading to drop. Using a calculation table, the reading from the dry thermometer and the reading drop from the wet thermometer are used to determine the relative humidity.

Other kinds of hygrometers use human hair (blond) to determine moisture content. These are called mechanical hygrometers, based on the principle that organic substances ( human hair) contract and expand in response to the relative humidity. The contraction and expansion moves a needle guage. In 1783, Swiss physicist and geologist, Horace Bénédict de Saussure built the first hygrometer using a human hair to measure humidity.

Some hygrometers use the measurements of changes in electrical resistance, using a thin piece of lithium chloride or other semiconductor devices and measuring the resistance which is affected by humidity.

Leonardo da Vinci built the first crude hygrometer in the 1400s. Francesco Folli invented a more practical hygrometer in 1664.

Robert Hooke
Robert Hooke invented the universal joint, the iris diaphragm, and an early prototype of the respirator; invented the anchor escapement and the balance spring, which made more accurate clocks possible, invented or improved meteorological instruments such as the barometer, anemometer, and hygrometer; and so on.

John Frederic Daniell
In 1820, British chemist and meteorologist, John Frederic invented a dew-point hygrometer, which came into widespread use. Daniel is best known for inventing the Daniell cell, an improvement over the voltaic cell used in the early history of battery development.

Hair Hygrometer
Build your own hair hygrometer.

How To Calibrate Your Hygrometer
How to calibrate your hygrometer (for use with cigar boxes).

Related Information
Other Weather Measuring  Instruments