A hygrometer UK /haɪˈɡrɒmɪtə/ is an instrument used for measuring the moisture content in the environment. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity. Modern electronic devices use temperature of condensation, or changes in electrical capacitance or resistance to measure humidity changes.The metal-paper coil hygrometer is useful for giving a dial indication of humidity changes. It appears most often in very inexpensive devices, and its accuracy is limited, with variations of 10% or more. In these devices, humidity is absorbed by a salt-impregnated paper strip attached to a metal coil, causing the coil to change shape. These changes (analogous to those in a bimetallic thermometer) cause an indication on a dial.
These devices use a human or animal hair under tension. The length of the hair changes with humidity and the length change may be magnified by a mechanism and/or indicated on a dial or scale. The traditional folk art device known as a weather house works on this principle.