Bimetallic strip

A bimetallic strip, also called bimetal strip could consist of a strip of brass and a strip of steel welded together. 

The strip is normal or does not experience expansion at some reference temperature.  

However, when the temperature rises above the reference temperature, the strip will bend.

When the strip is heated, brass expands more than iron. This expansion will make the strip bend toward the iron.

When the strip is cooled, brass contracts more than iron. This contraction will make the strip bend toward the brass.

Notice that the metal that expands the most (brass) is also the one that contracts the most (brass)

This back and forth movement of strip can be used in many applications such as to turn on/off a switch or regulate a valve. 

 Bimetallic strip in thermostat

A thermostat is a practical application of the bimetal strip and the bimetal strip is probably the most important piece in a thermostat.

A thermostat has 3 important main components.

• A bimetal strip
• A mercury switch
• Wires that connect to a furnace

Inside the thermostat, the bimetal strip is coiled up. When it is hot in the house, the bimetal strip tends to unwind or expand. As a result the magnet does not make contact with the mercury switch. When the magnet does not make contact with the mercury switch, the armature is down. When the armature is down, the contact points (small and green rectangle) do not touch. This breaks the circuit and no heat will flow in the house. 

However, when it is cold in the house, the bimetal strip tends to contract. As a result the magnet will make contact with the mercury switch. When the magnet makes contact with the mercury switch, the armature goes up. When the armature is up, the contact points ( small and green rectangle) will touch. This completes the circuit and heat will flow in the house.