The twisting motion of a heated helix is a result of a physical principle known as differential thermal expansion. This effect is demonstrated in a component called a bimetallic strip. By harnessing this behavior, many simple and reliable temperature-control devices can function without complex electronics. The change in temperature causes a physical movement, which can then be used to perform a mechanical action.
The Science of Bimetallic Strips
Nearly all materials exhibit thermal expansion, meaning they expand when heated and contract when cooled. This change occurs because heating increases the kinetic energy of a material’s atoms, causing them to move more vigorously and push each other farther apart. The extent to which a material expands for a given temperature change is determined by its coefficient of thermal expansion.
This difference is the foundation of a bimetallic strip. These strips are made by bonding two different metals, such as steel and brass, together side-by-side. Steel has a lower coefficient of thermal expansion compared to brass. When this composite strip is heated, the brass attempts to lengthen more than the steel.
Because the two strips are securely joined, they cannot expand independently. The greater expansion of the brass layer forces the entire strip to bend. The strip will curve with the brass on the outer, longer side of the arc and the steel on the inner, shorter side.
Why the Helix Shape Is Used
While a straight bimetallic strip demonstrates the principle of differential thermal expansion, the amount of bending produced by a short, straight strip is small. To generate a more pronounced and mechanically useful movement within a compact area, a very long strip is coiled into a helix or spiral shape.
When the coiled bimetallic strip is heated, the bending action occurs along its entire length. Instead of a simple curve, this cumulative bending forces the helix to either tighten or unwind. This produces a twisting or rotating motion at the free end of the coil. This rotational movement is more practical for operating switches or indicators than the simple bend of a straight strip.
Common Devices Using This Principle
The application of the twisting bimetallic helix is in mechanical thermostats. In these devices, the end of the helix is positioned to interact with an electrical switch. As the room temperature drops, the coil twists until it pushes a contact, completing a circuit and activating the heating system. As the room warms, the helix twists back, pulling away from the contact and breaking the circuit to shut the furnace off.
This principle is applied in various thermal switches. In a toaster, a bimetallic strip bends as it heats up, and when it reaches a certain temperature, it triggers a mechanism that cuts power and pops the toast up. Older car models used a bimetallic strip for their turn signal flashers. Current flowing through the strip heats it, causing it to bend and break the circuit, turning the light off. As it cooled, it would straighten, reconnect the circuit, and make the light flash again, creating the clicking sound.