Bimetallic strip

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This article is about the temperature-sensitive mechanical device. For metals composed of a mixture of two or more chemical elements, see alloy.




John Harrison's Memorial in Westminster Abbey, London




Mechanical clock mechanisms are sensitive to temperature changes which lead to errors in time keeping. A bimetallic strip is used to compensate for this in some mechanisms. The most common method is to use a bimetallic construction for the circular rim of the balance wheel. As the spring controlling the balance becomes weaker with increasing temperature, so the balance becomes smaller in diameter to keep the period of oscillation (and hence timekeeping) constant.



Thermostat with bimetal coil at (2)





A direct indicating dial thermometer (such as a patio thermometer or a meat thermometer) uses a bimetallic strip wrapped into a coil. One end of the coil is fixed to the housing of the device and the other drives an indicating needle. A bimetallic strip is also used in a recording thermometer. Breguet's thermometer consists of a tri-metallic helix.

Heat engines[edit]

Simple toys have been built which demonstrate how the principle can be used to drive a heat engine.[citation needed]

Electrical devices[edit]




Curvature of a Bimetallic Beam:

\kappa = \frac{6 E_1 E_2 (h_1 + h_2)h_1 h_2 \epsilon }{E_1^2 h_1^4 + 4 E_1 E_2 h_1^3 h_2 + 6 E_1 E_2 h_1^2 h_2^2 + 4 E_1 E_2 h_2^3 h_1 + E_2^2 h_2^4}

Where E_1 and h_1 are the Young's Modulus and height of Material One and E_2 and h_2 are the Young's Modulus and height of Material Two. \epsilon is the misfit strain, calculated by:

\epsilon = (\alpha_1-\alpha_2) \Delta T \,

Where α1 is the Coefficient of Thermal Expansion of Material One and α2 is the Coefficient of Thermal Expansion of Material Two. ΔT is the current temperature minus the reference temperature (the temperature where the beam has no flexure).[2][3]

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