Typically used to control a heating or cooling system, a thermostat is an electro-mechanical on/off switch that is activated by temperature changes. The sensing element is usually a spiral bimetallic strip that coils and uncoils in response to temperature changes because of differential expansion of the two bonded metals. In a mercury-switch thermostat, a ball of mercury rolls between contacts in one or more sealed glass ampoules, which are attached to a metal strip. The switch works when the mercury makes or breaks an electrical circuit, which creates a signal for heating or cooling from a furnace or central air conditioning system. Each glass ampoule contains approximately three grams of mercury, which is about the size of a dime. Mercury's unique properties -- high conductivity, high surface tension and liquidity at room temperature -- have made it a useful component in many electric switches.

To determine whether a thermostat contains mercury, carefully remove the front plate and look for anywhere from one to six small glass ampoules containing silver liquid. Because the mercury in thermostats is completely enclosed, it does not pose an immediate threat to health or to the environment, unless the glass ampoule breaks or the product is disposed of in the regular trash.

Mercury thermostats operate quietly, require little maintenance, and provide up to 30-40 years of service. According to the National Electrical Manufacturer's Association (NEMA), there are more than 50 million mercury-switch thermostats in homes across the United States