A hygrostat, or humidistat, is an instrument used to measure and control the relative humidity in an environment. Hygrostats are very similar to thermostats, but respond to humidity instead of temperature.
Modern devices use the temperature of condensation to measure differences in humidity. They can use changes in electrical capacitance or resistance to measure differences as well.
Hygrostats and humidistats are typically installed within enclosures and connected to enclosure heaters. If the humidity in the enclosure reaches a predefined set point, the hygrostat will turn on the heater to raise the condensation point. This helps protect electronic components inside the enclosure from damage caused by condensation and corrosion.