"A fire alarm in a simple form consists of a thermostat and a bell. The thermostat is composed of a bimetallic spring (shown in orange and gray) bent into a crescent. Copper is on the outside of the spring, and expands more than the steel on the inside. As the temperature goes up, the spring elongates and the copper bends the spring downward toward the contact point. When the temperature hits 130 degrees Fahrenheit, contact is made and the alarm bell rings. In the example above, touch your mouse to the mercury in the thermometer. Slowly raise the temperature until it is above 130 degrees. As the temperature rises, the bimetallic strip slowly elongates and bends downward toward the contact."
https://en.wikipedia.org/wiki/Bimetallic_strip
