How does an electric kettle switch itself off?

1. Water goes in, the lid is shut, and the switch is pressed down. Inside the switch, two pieces of metal touch, and that lets electricity flow.
2. Electricity flows through a heating element in the base. It is hard for electricity to get through, so the element gets very hot and heats the water. Warm water rises and cooler water sinks to take its place.
3. At 100 degrees Celsius the water boils. Bubbles of steam form at the hot bottom, rise and burst at the top. The space under the lid fills with hot steam.
4. With the lid shut, the steam finds a small opening near the top. It leads into a tube that runs down inside the handle to the switch. At the bottom waits a small metal disc.
5. The disc is made of two different metals stuck together. When they get hot, one grows more than the other, so the disc has to bend. Suddenly it snaps inside out and pushes the switch off.
6. The contacts in the switch are pulled apart, so electricity cannot flow. The element cools and the boiling stops. The kettle switched itself off, using nothing but its own steam.

Tap each part of the kettle and watch what it does.

• Heating element. The heating element sits under a flat metal plate at the bottom. Electricity makes it very hot, and the plate passes the heat to the water.
• Lid. The lid keeps the steam inside the kettle. That matters: with the lid shut, the steam has to go down the tube. With the lid open, it just floats away.
• Steam tube. A tube hidden in the handle carries steam from the top of the kettle down to the switch.
• Snap disc. This small disc is made of two metals stuck together. When steam heats it, one metal grows more than the other. The disc suddenly pops inside out and pushes the switch.
• Switch and contacts. Pressing the switch pushes two metal contacts together so electricity can flow. When the disc snaps, it pushes them apart and the electricity stops.
• Boil-dry guard. A second disc touches the bottom plate. If the kettle is switched on with no water, the plate gets too hot, this disc snaps and the power is cut.
• Power base and cord. The base stays plugged in on the counter. Round connectors in the middle pass electricity up into the kettle, so the kettle can be lifted off to pour.

Tap the switch to boil the water. Watch the steam, the disc and the contacts in the magnifier. Then try it with the lid open.

Experiment: You leave the lid open. What will the kettle do when the water boils?

1. With the lid shut, tap the switch and wait for the click.
2. Press Start again with cold water. Tap the lid to open it, then switch on.
3. Watch the steam and the disc. Close the lid and see what happens.

What we found: With the lid shut, steam is pushed down the tube, heats the disc, and the kettle clicks off a few seconds after boiling starts. With the lid open the steam escapes out of the top, the disc stays too cool to snap, and the kettle keeps boiling. The kettle senses steam, not the water.

Turn on JavaScript to try it yourself.

Three cups need a quarter liter each. Put in just enough water, make sure the kettle can switch itself off, and boil it.

An electric kettle has a heating element in its base. When you press the switch, electricity flows through the element, and because electricity struggles to get through it, the element gets very hot. The heat passes into the water. Warm water rises, cooler water sinks, and soon all the water is hot.

At 100 degrees Celsius the water boils and makes lots of steam. Here is the clever part: the kettle does not have a thermometer in the water. Instead, with the lid shut, the steam is pushed into a small tube that runs down inside the handle.

At the end of the tube sits a little disc made of two different metals stuck together. When metals get hot they grow a tiny bit, and these two grow by different amounts. That makes the disc want to bend. It holds its shape as long as it can, then suddenly pops inside out, like the click-button on a jam jar lid. A jar lid pops because you press it, though. The disc pops because of heat.

The popping disc pushes the switch. Two metal contacts inside are pulled apart, the electricity stops, and the kettle is off. That is why you should always shut the lid: without it, the steam floats away and the disc does not get hot. A second disc under the bottom plate switches the kettle off if someone turns it on with no water inside.

Key words

Heating element

The part that gets hot when electricity flows through it.

Circuit

A complete loop that electricity can flow around. A gap in the loop stops it.

Bimetallic disc

A disc made of two metals that bends or pops when it gets hot.

Thermal expansion

Things getting slightly bigger when they get hotter.

Contacts

The two pieces of metal in a switch that touch to let electricity flow and separate to stop it.

Check your thinking

1. What tells a kettle that the water is boiling?

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   Steam reaching a metal disc. Steam flows down a tube and heats the disc. The kettle senses the steam, not the time.

2. What happens inside the switch when the disc snaps?

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   Two contacts are pushed apart, so the electricity stops. Electricity needs a complete loop. Pulling the contacts apart makes a gap, so it cannot flow.

3. Why might a kettle keep boiling if its lid is left open?

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   The steam escapes from the top and does not go down the tube. The disc only gets hot if steam is pushed down the tube. An open lid lets the steam float away.

4. Why does the disc bend when it gets hot?

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   It is made of two metals, and one grows more than the other. Two metals stuck together cannot both stay flat if one grows more, so the disc bends and then pops.

5. You boil twice as much water in the same kettle from the same starting temperature. About how long does it take?

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   About twice as long. The energy needed is proportional to the mass of water, and the power is fixed, so the time doubles.