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.
Expand And Contract
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.
Controlling The Furnace
In a thermostat, the bimetallic coil is fastened firmly at one end. The other end is free to move. Its motion, as the temperature changes, can be used to control your furnace.
For example, if the coil tightens as it cools, the free end can curl toward a metal contact that starts the furnace. As the air heats up and the coil loosens, the free end curls away from the contact and the furnace shuts off.
When you adjust the thermostat you move the contact. When the contact is closer to the end of the coil, it will take a smaller drop in temperature to activate the furnace and your house will be warmer. Sliding the contact away will keep your house cooler.