hydrostat

from The Century Dictionary.

• noun An apparatus of any kind for preventing the explosion of steam-boilers.
• noun An electrical device for detecting the presence of water, used as a protection against damage to buildings from overflow or leakage.

from the GNU version of the Collaborative International Dictionary of English.

• noun A contrivance or apparatus to prevent the explosion of steam boilers.
• noun A device, usually electrical, for indicating or regulating the height of water in a reservoir or receptacle.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a device that automatically responds to drops in humidity by adding moisture to the air to maintain the humidity at a desired setting, similar to how a thermostat works for temperature.