The instrument used to measure humidity is known as a hygrometer. Hygrometers may be used to measure the relative humidity or absolute humidity in the air.
Hygrometers are commonly used to determine the humidity in areas where a specific humidity must be maintained. Some hygrometers operate by studying the contraction and expansion of hair when contacted by humidity. Some variations of hygrometers, such as psychrometers, operate by cooling the bulb of a thermometer through evaporation and determining the temperature of the air, while others function by measuring the resistance to an electric current, which changes with humidity. Chemical hygrometers determine humidity by absorbing moisture and measuring the weight of chemical materials after absorption.