hygrometer

A hygrometer (pronounced /haɪˈɡrɒmɪtər/) is an instrument used to measure the water vapor content in the atmosphere, soil, or enclosed spaces. Humidity measuring devices are generally based on measurements of other quantities such as temperature, pressure, mass, or a mechanical or electrical change in a substance as it absorbs moisture. Through calibration and calculation, the humidity can be determined from these measurements. Modern electronic devices use the condensation temperature (dew point) or changes in electrical capacitance or resistance to measure differences in humidity. The first simple hygrometer was invented in 1480 by the Italian Renaissance polymath Leonardo da Vinci; a more modern version was created in 1755 by the Swiss polymath Johann Heinrich Lambert. The maximum amount of water vapor that can be held in a given volume of air (saturation) varies greatly with temperature; cold air can hold less water per unit volume than hot air. Most instruments respond to, or are calibrated to measure, relative humidity (RH). Relative humidity is the amount of water vapor relative to the maximum amount of water vapor that can be held at a given temperature, expressed as a percentage.