A thermostat regulator has been constructed with a tungsten‐mercury contact in hydrogen permanently enclosed in glass. Adjustment is made by a mercury‐tight tungsten plunger in another 2 mm capillary, one complete turn of the micrometer head advancing the plunger approximately 1 mm, equivalent to 0.08°C. The contact breaks approximately 1 milliampere which actuates relays controlling the heating circuit of one ampere, 110 volts ac. The regulator kept a 50‐liter water thermostat constant to ±0.002°C, for an hour, with a 5‐minute regulation cycle. With an auxilary heater close to the regulator, the temperature variation was less than ±0.001°C over a period of three hours and the regulation cycle less than 1 minute.