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The Amplification of Thermal Expansion/Contraction

G.R.Dixon, 8/9/2005

The coefficient of thermal expansion (COTP) is an indicator of how much a given material expands/contracts with changes in its temperature. Many metals have relatively high COTPs. The expansion/contraction occurs at the atomic level, and in any macroscopic sample the potential force exerted during such expansion/contraction might loosely be described as a net "quantum force." Although usually acting through relatively small distances, such forces can have considerable magnitudes.

Bonding thin and narrow sheets of two different metals (with different COTPs) along their inner faces forms a bimetallic strip (BMS). When the strip is heated/cooled, the half with the higher COTP expands/contracts more than the other half, causing the strip to bend away from its nominal (typically room temperature) straight form. BMSs have long been used in thermostats, opening and closing control circuits for heaters and air conditioners by tilting mercury switches (doing mechanical work in the process).

Fig. 1 depicts a series of BMSs, joined saw-tooth-fashion at their ends. Joining several strips together as indicated magnifies the displacement of end "A". The source of heating and cooling could be obtained by alternately exposing the blackened tops of the strips to sunlight and shade (with the straight configuration existing at a median temperature). As the set of strips is alternately heated and allowed to cool, "A" should move back and forth piston-fashion, being able to do mechanical work in the process. The power would of course be modest compared to that realizable from an internal combustion engine. But, it is green power. And the strips should operate unattended for many years. Arrays of such strips in parallel would magnify the force (and thus delivering greater power) at their joined "A" ends.

Figure 1

Magnifying BMS Displacement and Force