Bimetallic Strip and Thermostat
A bimetallic strip is two different metals joined into one strip. Heat it or cool it, and it bends. That simple bend can switch a furnace on and off.
In this lesson, you will learn
- what a bimetallic strip is
- why the strip bends when it is heated or cooled, and which way it bends
- how a thermostat uses a bimetal strip to turn the heat on and off
What Is a Bimetallic Strip?
A bimetallic strip, also called a bimetal strip, could consist of a strip of brass and a strip of steel welded together.
The strip is straight at some reference temperature.
However, when the temperature rises above or falls below the reference temperature, the strip will bend.
A bimetallic strip is at its reference temperature. What does it look like?
Show answer
It is straight. It only bends when the temperature goes above or below the reference temperature.
Why the Strip Bends When It Is Heated or Cooled
When the strip is heated, brass expands more than steel. This expansion will make the strip bend toward the steel.
When the strip is cooled, brass contracts more than steel. 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).
The two metals are welded together, so they can't slide past each other. When one metal gets longer than the other, the only way the strip can fit both lengths is to curve. The longer metal goes on the outside of the curve, like the outside lane of a running track.
When the strip is heated, the metal that expands more is on the outside of the curve. When the strip is cooled, it is on the inside of the curve.
How much different metals expand
Each metal expands by its own amount. The table shows how much a rod 1 meter long gets longer when it is heated by 100 °C.
| Metal | Extra length |
|---|---|
| Aluminum | about 2.3 mm |
| Brass | about 1.9 mm |
| Copper | about 1.7 mm |
| Steel | about 1.2 mm |
| Invar (a nickel-iron alloy) | about 0.1 mm |
A bigger difference between the two metals makes the strip bend more for the same change in temperature.
- Use the table: copper grows about 1.7 mm per meter, and steel grows about 1.2 mm per meter. Copper expands more.
- When the strip is heated, the metal that expands more ends up longer, so it goes on the outside of the curve.
- Copper is on the outside, and steel is on the inside.
The strip bends toward the steel.
Thinking that a heated strip bends toward the metal that expands more. It is the other way around: the metal that expands more becomes the longer side, and the longer side is the outside of the curve. So a heated strip bends away from it, toward the other metal.
A strip of aluminum and steel is cooled. Which way does it bend?
Show answer
Toward the aluminum. Aluminum contracts more, so it becomes the shorter side, on the inside of the curve.
This back-and-forth movement of the strip can be used in many applications, such as to turn a switch on or off or to regulate a valve. Thermostats, circuit breakers, dial thermometers, toasters and electric kettles all use a bimetal strip.
Bimetallic Strip in a 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 main components:
- A bimetal strip
- A mercury switch
- Wires that connect to a furnace
When it is warm in the house
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 green and red rectangles) do not touch. This breaks the circuit, and no heat will flow in the house.
When it is cold 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 green and red rectangles) will touch. This completes the circuit, and heat will flow in the house.
Mercury is poisonous. The big US thermostat makers stopped making mercury thermostats in 2009, and many states now ban selling them. An old mercury thermostat must be recycled, never thrown in the trash. Most new thermostats are electronic.
The house gets colder. Does the circuit to the furnace open or close?
Show answer
It closes. The strip contracts, the magnet touches the mercury switch, the armature goes up and the contact points touch, so the furnace turns on.
Summary
- A bimetallic strip is two different metals, such as brass and steel, welded together.
- At its reference temperature the strip is straight. Above or below that temperature, it bends.
- Heated: the metal that expands more is on the outside, so the strip bends toward the other metal.
- Cooled: the metal that contracts more is on the inside, so the strip bends toward it.
- In a thermostat, the coiled strip moves a magnet that closes the circuit when the house is cold and opens it when the house is warm.
Test Yourself: 10-Question Quiz
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