text: The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases steel and brass. The strips are joined together throughout their length by riveting, brazing or welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled. The sideways displacement of the strip is much larger than the small lengthways expansion in either of the two metals.
  In some applications, the bimetal strip is used in the flat form. In others, it is wrapped into a coil for compactness. The greater length of the coiled version gives improved sensitivity.
  The radius of curvature  of a bimetallic strip depends on temperature  according the formula derived by French physicist Yvon Villarceau in 1863 in his research for improving the precision of clocks: 
  where   is the total thickness of the bimetal and  is the coefficient of thermal expansion and  is the thickness. The formula can also be rewritten as a function of the thermal misfit strain . And if the modulus and height are similar, we simply have .
  An equivalent formula can be derived from the beam theory.