Thermostat - bi-metallic (mercury bulb)

Air Conditioning and Heat Pumps · Thermostats · printed page 224 (PDF page 225)

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Mercury Bulb Mercury Bulb – Air Conditioning

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
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
circuit open
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 side-by-side diagrams show the operation of a bi-metallic coil thermostat with a mercury bulb switch. On the left, warming the coil tips the bulb so mercury moves away from the electrical contacts, opening the circuit to the gas valve or oil burner. On the right, cooling the coil tips the bulb toward the contacts, allowing mercury 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. Both diagrams illustrate normal operating states rather than defects.

Components

thermostat bi-metallic coil mercury bulb switch electrical contacts central hub temperature setting dial connection electrical circuit leads

Materials

mercury bimetallic strip

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