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The red circle above shows where a glass bulb of mercury is located within a heating / cooling thermostat. Mercury has excellent electrical properties; when a heat sensitive circular spring within the thermostat either “coils up” or “coils down”, the mercury in the glass bulb either tips up or down. When the glass bulb tips down, the liquid mercury covers two exposed electrodes, causing the circuit to close. Once this circuit is closed, the furnace turns on until “called upon” by the thermostat to turn off (by tipping up), opening up the electrical circuit. Some thermostats contain as many as 4 separate mercury-containing bulbs.