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 constants of 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 generating device.

For an illustration of a bi-metal element in a simple thermostat, see the thermostat 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.