Bi-metal

''This page concerns the temperature-sensitive mechanical device. For bi-metal electrical devices see ''thermocouple'' and ''Peltier-Seebeck effect. ''For metals composed of a mixture of two or more metallic elements see'' alloy. ==Fundamentals== A bi-metallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two layers, usually iron and copper. The two layers are joined together to form the strip. Owing to the difference in the coefficient_of_thermal_expansion of the two materials, a flat strip will bend one way (toward the iron part) if heated, and in the opposite direction if cooled below its normal temperature. In some applications the bi-metal strip is used in the flat form. In others, it is wrapped into a coil, which gives greater sensitivity in a compact space. ==Applications== ===Thermostats=== In regulating thermostats that operate over a wide range of temperatures the bi-metal strip is mechanically fixed and attached to an electrical power source while the other (moving) 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 (as in a heater control) or it may close a contact (as in a refrigerator or air conditioner. The electrical contacts may control the power directly (as in a household iron) 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 (a small, continuously burning flame). In devices without pilot lights for ignition (as in most modern gas clothes dryers and some natural gas heaters and decorative fireplaces) 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 gap generating device. For an illustration of a bi-metal element in a simple thermostat, see the thermostat#Simple Two Wire Thermostats entry. ===Thermometers=== A direct indicating dial thermometer (such as a patio thermometer or a meat thermometer) uses a bi-metallic strip wrapped into a coil, as does a common household thermostat. One end of the coil is fixed to the chassis of the device and the other is connected to an indicating needle. HVAC

Bi-metal

Change: spark igniter was changed to a piezo type, but these are not thermostat controlled. I would contact the contributor directly, but this was an unregistered user. Expanded to become an electricaly powered spark igniter for decorative fireplaces. If the contributor can back up the piezo ignition in gas dryers, please add explicity and notate in this page. Being a registered user has its advantages, please register.

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B

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Words begining with Bi-metal:

Bi-metal
Bi-metal
Bi-metalic
Bi-metalic_strip
Bi-Metallic_Investment_Co._v._State_Board_of_Equalization
Bi-metallic_strip
Bi-metal_strip