Editing Hygrometer (section)

{{Short description|Instrument for measuring humidity}}
{{Distinguish|hydrometer}}
[[image:Haar-Hygrometer.jpg|thumb|A hair tension dial hygrometer with a nonlinear scale.]]
{{Humidity}}
A '''hygrometer''' is an instrument that measures [[humidity]]: that is, how much [[water vapor]] is present.<ref>This is different from measuring the ''moisture content'' of something non-gaseous like soil, where liquid water is part of the measurement.</ref> Humidity measurement instruments usually rely on measurements of some other quantities, such as temperature, pressure, mass, and mechanical or electrical changes in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can be used to indicate the humidity. Modern electronic devices use the temperature of condensation (called the [[dew point]]), or they sense changes in electrical [[capacitance]] or [[Electrical resistance|resistance]].

The maximum amount of water vapor that can be present in a given volume (at [[Relative humidity|saturation]]) varies greatly with temperature; at low temperatures a lower mass of water per unit volume can remain as vapor than at high temperatures. Thus a change in the temperature changes the relative humidity.

A prototype hygrometer was invented by [[Leonardo da Vinci]] in 1480. Major improvements occurred during the 1600s; [[Francesco Folli]] invented a more practical version of the device, and [[Robert Hooke]] improved a number of meteorological devices, including the hygrometer. A more modern version was created by Swiss polymath [[Johann Heinrich Lambert]] in 1755. Later, in the year 1783, Swiss physicist and geologist [[Horace Bénédict de Saussure]] invented a hygrometer that uses a stretched human hair as its sensor.

In the late 17th century, some scientists called humidity-measuring instruments ''hygroscopes''; that word is no longer in use, but ''hygroscopic'' and ''[[hygroscopy]]'', which derive from it, still are.