Steam that flips a switch
Why Does a Kettle Switch Itself Off?
Steam is piped down to a small strip of two metals stuck together, and it bends far enough to snap the power off.
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Step 1 of 5
The element heats the water, not the air.
A flat plate in the base gets hot and passes that heat straight into the water touching it. Warm water rises, cold sinks, and the whole lot stirs itself.
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Step 2 of 5
At boiling point the temperature stops climbing.
Once it reaches 100°C the extra heat stops raising the temperature and starts tearing molecules out of the liquid instead. That is what steam is.
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Step 3 of 5
Steam is the signal.
A tube runs from the spout area down inside the handle. It carries almost no steam until the water actually boils — and then it carries plenty.
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Step 4 of 5
Two metals glued together bend when they get hot.
Different metals swell by different amounts. Bond a strip of one to a strip of another and heating it forces the whole strip to curve.
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Step 5 of 5
The bend snaps the switch open.
The disc does not creep. It holds its shape until a threshold and then flips all at once, kicking the contacts apart with an audible click.
A 3 kW kettle draws about 13 amps — close to the limit of a British plug, and the reason kettles are rarely wired to anything else.
The short version
A kettle switches off because steam bends a strip of two bonded metals, and past a threshold that strip snaps the circuit open.
Try it yourself
Listen for the click just after the steam appears — not before. The kettle is waiting for real steam, not for a timer.
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