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hygrometer
A device to measure humidity (the amount of water vapor the air holds). Usually, hygrometers measure relative humidity, i.e., the amount of moisture in the atmosphere as a percentage of the saturation level at that temperature and pressure.
The hair hygrometer, though of limited accuracy, is common. The length of a hair increases with increase in relative humidity. This length change is amplified by a lever and registered by a needle on a dial.
The wet and dry bulb hygrometer (psychrometer) has two thermometers mounted side by side, the bulb of one covered by a damp cloth. Air is moved across the apparatus (e.g., by a fan) and evaporation of water from the cloth draws latent heat from the bulb. Comparison of the two temperatures, and the use of tables, gives the relative humidity.
The dewpoint hygrometer comprises a polished container cooled until the dew point is reached; this temperature gives a measure of relative humidity.
The electric hygrometer measures changes in the electrical resistance of a hygroscopic (water-absorbing) strip.
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