A Moment of Science

Inside Your Thermostat: How Two Pieces Of Metal Work Together

You may have looked at your thermostat and saw a piece of metal. Learn how the metal strip controls them temperature of your house.

a home thermostat

Photo: Mike Miller (flickr)

A fancy, electronic thermostat.

If you pull the cover off the thermostat in your living room, you may find a thin strip of metal. Look closely at the strip and you may find a clue as to how that strip turns the heat or air conditioning on and off.

How The AC Works

The two sides are probably different colors. That’s because the metal strip is really made of two different metals that cause the strip to bend when it changes temperature.

All metals expand when heated and contract when they cool. The most familiar example is the mercury in a thermometer. But solid metals, like iron and brass also expand and contract with the temperature. The difference is that some metals expand much more than others even with the same change in temperature.

What Are The Two Metals?

The two metals, usually brass and iron are welded or riveted together to make one strip. That strip is a switch that works like any other switch by connecting two points to complete an electrical circuit.

But the fact that the brass reacts more to temperature than iron does enables the switch to open and close in response to temperature.

As your house gets cold, the brass side contracts more than the iron pulling the strip toward the brass side. If the strip bends far enough, it touches an electrical contact, completing the circuit, and turning on the heater.

As the house warms up, the brass expands, pushing the strip away from the electrical contact and shutting off the heat.

Finding The Metal Strip

Some thermostats don’t use the two part metal strip. And if your’s is one of these, you probably have just such a strip somewhere else. Perhaps in your oven thermometer, your electric toaster, or the automatic choke on your car.

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