Bimetallic Strips: Working Principle and Applications

A bimetallic strip is made by bonding two different metals with different rates of thermal expansion face to face. When the temperature changes, one metal expands more than the other, so the strip cannot stay straight and bends into a curve. Cooling makes it bend the opposite way. This simple bending action works as a temperature-sensitive switch.

Bimetallic strips are everywhere: in room thermostats, electric irons, toasters, and circuit breakers. They need no battery or electronics, respond directly to temperature, and can open or close an electrical contact millions of times reliably. Understanding them shows how thermal expansion can be turned into useful mechanical work.

Key Concepts

The metals must have clearly different coefficients of linear expansion (α), for example invar with very low α bonded to brass or steel with higher α. The strip bends toward the metal with lower expansion on heating, because the high-expansion side becomes longer and forms the outside of the curve. Sensitivity increases with length, thinness, and the difference in α. The bond must be strong, often by welding or riveting, so the metals cannot slide past each other.

How It Works

1. At the reference temperature the bonded strip is straight and a contact is closed, allowing current to flow to a heater.
2. As temperature rises, the high-expansion metal tries to become longer than its partner.
3. Because they are bonded, the strip bends, with the high-expansion metal on the convex outer side.
4. Bending moves a contact apart, switching the heater off.
5. On cooling, expansion reverses, the strip straightens or bends the other way, remaking the contact and restarting heating, maintaining a steady temperature.

Worked Example

A strip uses brass (α ≈ 19 × 10⁻⁶ K⁻¹) bonded to invar (α ≈ 1.5 × 10⁻⁶ K⁻¹). For a temperature rise of 50 K, free brass would expand fractionally by 19 × 10⁻⁶ × 50 = 9.5 × 10⁻⁴, while invar expands only 1.5 × 10⁻⁶ × 50 = 0.75 × 10⁻⁴. The difference, about 8.75 × 10⁻⁴, over a 10 cm strip corresponds to roughly 0.09 mm of length mismatch. Bonded together, that mismatch is converted into visible bending of several millimetres at the free end, enough to snap open a contact in a thermostat and interrupt several amperes safely.

Common Misconceptions

• The strip bends because one metal melts or softens. No melting occurs; bending is purely from unequal expansion while both remain solid.
• Both metals expand equally. If α values were equal there would be no bending; the difference is the entire operating principle.
• It bends toward the high-expansion metal on heating. Actually it bends away; the longer high-expansion side becomes the outside of the curve.

Real-World Applications

Household thermostats, iron temperature controls, oven regulators, and toaster timers use bimetallic strips. In electrical safety, circuit breakers use the heat from excess current to bend a strip and trip the switch. Car indicator flashers, fire alarms, and greenhouse vent openers also rely on this dependable, power-free temperature sensing and actuation.

A bimetallic strip converts temperature change into bending by joining metals of unequal expansion. That bending makes and breaks circuits, giving simple, robust temperature control in countless home and industrial devices.