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 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. 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.
History
The earliest surviving bimetallic strip was made by the eighteenth-century clockmaker
John Harrison who is generally credited with its invention. He made it for his third marine
chronometer of 1759 to compensate for temperature-induced changes in the balance spring. It should not
be confused with his bimetallic mechanism for correcting for thermal expansion in the
gridiron pendulum. His earliest examples had two individual metal strips joined by rivets
but he also invented the later technique of directly fusing molten brass onto a steel
substrate. A strip of this type was fitted to his last timekeeper, H5. Harrison's invention
is recognized in the memorial to him in Westminster Abbey, England.
Applications Clocks
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 constant.
Thermostats
In the regulation of heating and cooling, thermostats that operate over a wide range
of temperatures are used. In these, one end of the bimetal strip is mechanically fixed
and attached to an electrical power source, while the other end carries an electrical
contact. In adjustable thermostats another contact is positioned with a regulating knob
or lever. The position so set controls the regulated temperature, called the set point.
Some thermostats use a mercury switch connected to both electrical leads. The angle of the
entire mechanism is adjustable to control the set point of the thermostat.
Depending upon the application, a higher temperature may open a contact or it may close a contact.
The electrical contacts may control the power directly or indirectly, switching electrical
power through a relay or the supply of natural gas or fuel oil through an electrically operated
valve. In some natural gas heaters the power may be provided with a thermocouple that is
heated by a pilot light. In devices without pilot lights for ignition the power for the
contacts is provided by reduced household electrical power that operates a relay controlling
an electronic ignitor, either a resistance heater or an electrically powered spark generating
device. Thermometers
A direct indicating dial 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
Simple toys have been built which demonstrate how the principle can be used to drive a heat
engine. Electrical devices
Bimetal strips are used in miniature circuit breakers to protect circuits from excess current.
A coil of wire is used to heat a bimetal strip, which bends and operates a linkage that unlatches
a spring-operated contact. This interrupts the circuit and can be reset when the bimetal
strip has cooled down. Bimetal strips are also used in time-delay
relays, lamp flashers, and fluorescent lamp starters. In some devices the current running
directly through the bimetal strip is sufficient to heat it and operate contacts directly.
Calculations Curvature of a Bimetallic Beam:
Where and are the Young's Modulus and height of Material One and and are the Young's Modulus
and height of Material Two. is the misfit strain, calculated by:
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. See also
Nitinol - a shape-memory alloy External links
Video of a circular bimetalic wire powering a small motor with iced water. Accessed February
2011. Notes
^ Sobel, Dava. Longitude. London: Fourth Estate. p. 103. ISBN 0-00-721446-4. "One of the
inventions Harrison introduced in H-3... is called... a bi-metallic strip."
^ Clyne, TW. “Residual stresses in surface coatings and their effects on interfacial
debonding.” Key Engineering Materials. Vol. 116-117, pp. 307-330. 1996
^ Timoshenko, J. Opt. Soc. Am. 11, 233