What is the X in X-mas?
A device used for regulating the relative humidity of an enclosed space. A hygrostat contains a sensor that detects a preset level of humidity and is connected to a heater that is turned off and on as needed in order to keep the humidity at the preset level. Hygrostats are used in humidifiers and dehumidifiers, and are placed in the ductwork of laboratories and other facilities where climate control is necessary. Also called humidistat.