0743 Thermostat Bi Metallic Mercury Bulb - Heating - Gas Burners -

Image Text: Furnace off > the central hub of the bi-metallic coil is attached to the thermostat temperature setting dial - turning the dial down (counterclockwise) means that it will have to get even cooler before the mercury rolls down and closes the contacts > when the coil warms up, it wants to straighten out - this tips the mercury switch so that the blob of mercury rolls away from the contacts, opening the circuit > electrical circuit to gas valve or oil burner > circuit open > bi-metallic coil > Furnace on > as the bi-metallic coil cools, the mercury moves toward the contacts - closing the circuit and starting the furnace > blob of mercury > contacts > circuit closed > bi-metallic coil