Hygrometer measures humidity in the air. There are more elaborate hygrometers, as well as devices called psychrometers, available for measuring humidity, but the most common and simplest hygrometer used in homes today measures changes in humidity using the same principle as does a single human hair when it absorbs or loses moisture.

History
The inventor of the first hygrometer was Nicholas of Cusa, a German cardinal and philosopher who, in 1450, experimented with placing some sheep's wool on a set of scales and then monitored the change in the wool's weight as it absorbed moisture from the air, or lost moisture on a dry day. He rightly concluded that the amount of water vapor in the air changed and that the change could be measured.
Features
A simple hygrometer uses the change in length of a human hair--or other organic fiber--to move a pointer across a calibrated dial. As the hair or fiber lengthens in damp air, the pointer gives a corresponding reading in relative humidity. As the hair becomes drier due to a lack of humidity in the air, it shrinks and moves the pointer as well.
Cost and Availibility
Mechanical hygrometers vary in price from $7 to $30, while electronic hygrometers range from $15 to $60 and higher, depending on quality. Hygrometers can be purchased in hardware stores, department stores, garden centers--usually anywhere thermometers are sold.
Considerations
A hygrometer should be placed in an area that won't be exposed to direct heat. Allow at least two hours for the hygrometer to register the relative humidity in a room.