Thermostat - bi-metallic (mercury bulb)

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Thermostat

Text in the illustration

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
electrical circuit to gas valve or oil burner
circuit open
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
bi-metallic coil
blob of mercury
contacts
as the bi-metallic coil cools, the mercury moves toward the contacts - closing the circuit and starting the furnace
circuit closed

Generated explanation

Two diagrams show the operation of a bi-metallic coil thermostat with a mercury bulb switch. On the left, warming causes the coil to straighten and tip the bulb so mercury moves away from the contacts, opening the electrical circuit to the gas valve or oil burner. On the right, cooling tips the bulb in the opposite direction, moving mercury toward the contacts to close the circuit and start the furnace. The central hub connects to the temperature-setting dial; turning the dial down counterclockwise requires a lower temperature before the contacts close.

Components

thermostat bi-metallic coil central hub mercury bulb switch electrical contacts electrical circuit leads

Materials

bi-metallic strip mercury

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