The type of fixed-temperature heat detector that uses two types of metal with different heat-expansion ratios is a bimetallic heat detector.
Bimetallic heat detectors work based on the principle of differential thermal expansion of two metals. A bimetallic strip, made up of two metals with different coefficients of thermal expansion, is used as a sensing element. When the temperature of the bimetallic strip increases, the metal with the higher coefficient of thermal expansion expands more than the other metal, causing the strip to bend. The bending of the strip activates a switch that signals the presence of heat.
Bimetallic heat detectors are commonly used in environments where a fixed-temperature detection is needed, such as in kitchens, boiler rooms, and other areas where heat may be present.