Thermometers just tell you the temperature. Thermostats need to do more than that. In addition to reading the air temperature, thermostats also turn your furnace on if the air gets too cool, and turn it off again when the air heats up. How do they do this? At the heart of many household thermostats is a type of thermometer called a bimetallic coil. This is made from two long, thin strips of metal, usually brass and steel, that have been joined firmly together, face to face, like a metallic sandwich. This new strip, brass on one side and steel on the other, is then coiled into a spiral. Remember that most substances expand when they’re heated and contract when they’re cooled. It’s also true that different substances expand and contract at different rates. In a bimetallic coil, the brass expands and contracts a little bit more than the steel does over the same range of temperature. For example, if the temperature rises, the brass side of the strip will lengthen slightly more than the steel side. This makes the coil tighten and loosen with changing temperature.