- Issue Time
- Aug 4,2017
Thisarticle is about the temperature-sensitive mechanical device. For metalscomposed of a mixture of two or more chemical elements.
Diagramof a bimetallic strip showing how the difference in thermal expansion in thetwo metals leads to a much larger sideways displacement of the strip
Abimetallic coil from a thermometer reacts to the heat from a lighter, byuncoiling and then coiling back up when the lighter is removed.
Abimetallic strip is used to convert a temperature change into mechanicaldisplacement. The strip consists of two strips of different metals which expandat different rates as they are heated, usually steel and copper, or in somecases steel and brass. The strips arejoined together throughout their length by riveting,brazing or welding. Thedifferent expansions force the flat strip to bend one way if heated, and in theopposite direction if cooled below its initial temperature. The metal with thehigher coefficient of thermal expansion is on the outer side of the curve whenthe strip is heated and on the inner side when cooled.
Thesideways displacement of the strip is much larger than the small length waysexpansion in either of the two metals. This effect is used in a range of mechanicaland electrical devices. In some applications the bimetal strip is used in theflat form. In others, it is wrapped into a coil for compactness. The greaterlength of the coiled version gives improved sensitivity.
John Harrison's Memorial in
Theearliest surviving bimetallic strip was made by the eighteenth-centuryclockmaker John Harrison who is generally credited with its invention. He madeit for his third marine chronmeter of1759 to compensate for temperature-induced changes in the balance spring Itshould not be confused with his bimetallic mechanism for correcting for thermalexpansion in the gridiron pendulum. His earliest examples had two individualmetal strips joined by rivets but he also invented the later technique ofdirectly fusing molten brass onto a steel substrate. A strip of this type wasfitted to his last timekeeper, H5. Harrison's invention is recognized in thememorial to him in Westminster Abbey,
Mechanicalclock mechanisms are sensitive to temperature changes which lead to errors intime keeping. A bimetallic strip is used to compensate for this in somemechanisms. The most common method is to use a bimetallic construction for thecircular rim of the balance wheel. As the spring controlling the balancebecomes weaker with increasing temperature, so the balance becomes smaller indiameter to keep the period of oscillation (and hence timekeeping) constant.
Inthe regulation of heating and cooling, Thermostats that operate over a widerange of temperatures are used. In these, one end of the bimetal strip ismechanically fixed and attached to an electrical power source, while the other(moving) end carries an electrical contact. In adjustable thermostats another contactis positioned with a regulating knob or lever. The position so set controls theregulated temperature, called the setpoint .
Somethermostats use a mercury switch connected to both electrical leads. The angleof the entire mechanism is adjustable to control the set point of thethermostat.
Dependingupon the application, a higher temperature may open a contact (as in a heatercontrol) or it may close a contact (as in a refrigerator or air conditioner).
The electrical contacts may control the power directly (as in ahousehold iron) or indirectly, switching electrical power through a relay or the supply of natural gas or fuel oil through an electrically operated valve. Insome natural gas heaters the power may be provided with a thermocouple that isheated by a pilot light (a small, continuously burning, flame). In deviceswithout pilot lights for ignition (as in most modern gas clothes dryers andsome natural gas heaters and decorative fireplaces) the power for the contactsis provided by reduced household electrical power that operates a relaycontrolling an electronic ignitor, either a resistance heater or anelectrically powered spark generating device.
A direct indicating dial Thermostat (such as a patio thermometer or ameat thermometer) uses a bimetallic strip wrapped into a coil. One end of thecoil is fixed to the housing of the device and the other drives an indicatingneedle. A bimetallic strip is also used in a recording thermometer. Breguet’sthermometer consists of a tri-metallic helix.
Simpletoys have been built which demonstrate how the principle can be used to drive aheat engine.
Bimetalstrips are used in miniature cricuit breakers to protect circuits from excesscurrent. A coil of wire is used to heat a bimetal strip, which bends andoperates a linkage that unlatches a spring-operated contact. This interruptsthe circuit and can be reset when the bimetal strip has cooled down.
Bimetalstrips are also used in time-delay relays, lamp flashers, and fluorescent lampstarters. In some devices the current running directly through the bimetalstrip is sufficient to heat it and operate contacts directly.