Do you feeling too hot

You'll be wanting to cool down, then. Feeling too cold? You'll need to warm up. Our bodies are amazing, self-regulating mechanisms that can constantly adjust to keep their temperature within a whisker of 37°C (98.6°F). But the rest of the world isn't quite so helpful. If we want our homes to keep their temperature more or less constant, we have to keep switching our heaters on and off—or, alternatively, rely on clever gadgets called thermostats to do the job for us. What are they and how do they work? Let's take a look inside!

What is a thermostat?
You might have a temperature control on a wall in your home to control the heating system but, although it's probably marked in degrees, it's not a thermometer. It's called a thermostat, a modern word based on two ancient Greek ones: thermo (meaning heat) and statos (which means standing and is related to words like stasis, status quo, and static—meaning to stay the same). We can tell just from its name that a thermostat is something that "keeps heat the same": when our home is too cold, the thermostat switches on the heating so things quickly warm up; once the temperature reaches the level we've set, the thermostat switches the heating off so we don't boil.
Let's just be clear about the difference: a thermometer is something that measures the temperature; a thermostat is something that tries to maintain the temperature (keep it roughly the same).
 

How thermostats work
So how does a thermostat work? Most things get bigger when they heat up and smaller when they cool down (water is a notable exception: it expands when it heats up and when it freezes too). Mechanical thermostats use this idea (which is called thermal expansion) to switch an electric circuit on and and off. The two most common types use bimetallic strips and gas-filled bellows.