How does a thermostat regulate temperature?

How does a thermostat regulate temperature? — TF, Auburn, WA

A typical thermostat turns on the furnace whenever the temperature falls below a certain temperature and turns the furnace off whenever the temperature rises above another temperature. Those two temperatures are slightly separated so that the furnace doesn’t turn on and off too rapidly. In a typical home thermostat, a bimetallic coil tips a small mercury-filled glass bottle. The bimetallic coil is made from two different metal strips that have been sandwiched together and then rolled into a coil. As the temperature changes, the two metals expand differently and the coil winds or unwinds. As it does, it tips the glass bottle and the mercury rolls from one end of the bottle to the other. When the mercury falls to one end, it allows an electric current to flow between two wires and the furnace turns on. When the mercury falls to the other end of the bottle, the current stops flowing and the furnace turns off. So the winding and unwinding of the coil controls the furnace and the home temperature tends to hover at the point where the bottle of mercury is almost perfectly level. When you adjust the set point of the thermostat, you tilt the whole coil and bottle so that the average temperature in your home must shift in order for the bottle to be almost level.

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