Abstract: There is disclosed a mercury switch for use in thermostat applications. The switch has a hollow housing fabricated from glass and is positioned in the horizontal plane when in an operating orientation. The switch has a first common electrode within said housing and extending parallel to the horizontal plane with a terminal end extending from the housing. Positioned above the first electrode is a second shorter electrode with a third longer electrode positioned above said second electrode said third electrode being longer than said second but shorter than said first. Positioned above the third electrode is a longer fourth electrode. Essentially, major portions of the electrodes within the housing are parallel to each other. A charge of mercury is in the housing and operates to make contact between the electrodes in four operating modes indicative of thermostat operation for control of a heat pump, furnace and air conditioner.