A bimetallic strip is used to convert a temperature change into mechanical displacement. 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 brass instead of copper. 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. This effect is used in a range of mechanical and electrical devices. 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.
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“Perfect present has no existence in our consciousness. As I said years ago in Erewhon, it lives but upon the sufferance of past and future. We are like men standing on a narrow footbridge over a railway. We can watch the future hurrying like an express train towards us, and then hurrying into the past, but in the narrow strip of present we cannot see it. Strange that that which is the most essential to our consciousness should be exactly that of which we are least definitely conscious.”
—Samuel Butler (1835–1902)