This article is about thetemperature-sensitive mechanical device. For metals composed of a mixture oftwo or more chemical elements.
Diagramof a bimetallic strip showing how the difference in thermal expansion in thetwo metals leads to a much larger sideways displacement of the strip
Abimetallic coil from a thermometer reacts to the heat from a lighter, byuncoiling and then coiling back up when the lighter is removed.
Abimetallic strip is used to convert a temperature change into mechanicaldisplacement. The strip consists of two strips of different metals which expandat different rates as they are heated, usually steel and copper, or in somecases steel and brass. The strips arejoined together throughout their length by riveting,brazing or welding. Thedifferent expansions force the flat strip to bend one way if heated, and in theopposite direction if cooled below its initial temperature. The metal with thehigher coefficient of thermal expansion is on the outer side of the curve whenthe strip is heated and on the inner side when cooled.
Thesideways displacement of the strip is much larger than the small length waysexpansion in either of the two metals. This effect is used in a range of mechanicaland electrical devices. In some applications the bimetal strip is used in theflat form. In others, it is wrapped into a coil for compactness. The greaterlength of the coiled version gives improved sensitivity.