Bimetallic Strip — Principle and Applications

As the name suggests, a bimetallic strip is made by securely joining two strips of different metals together (usually by riveting, brazing, or welding) along their entire length.

The most common combination used in physics labs is Brass and Iron (or Brass and Invar).

The strip works on the principle that different metals expand at different rates when heated (they have different coefficients of linear expansion).

1. When Heated: If the strip is heated, the metal with the higher expansion rate (e.g., Brass) expands more and becomes longer than the other metal (e.g., Iron). Because they are glued together, the strip is forced to bend in a curve. The metal that expands more (Brass) forms the outer, longer edge of the curve.
2. When Cooled: If the strip is cooled below room temperature, the brass contracts more than the iron. The strip will bend in the opposite direction, with the brass on the inside of the curve.

Because it bends at specific temperatures, the bimetallic strip is used as a mechanical switch:

1. Thermostats (Electric Irons & ACs): When the iron reaches the desired hot temperature, the strip bends and breaks the electrical circuit, turning off the heater. When it cools, it straightens, reconnecting the circuit.
2. Fire Alarms: The heat from a fire causes the strip to bend, touch a contact point, and complete a circuit that rings the alarm bell.
3. Circuit Breakers (MCBs): If too much current flows, the wire heats up. The bimetallic strip bends due to the heat, tripping the switch and cutting off the dangerous current.