Science Puzzle
How Does a Kettle Know When to Switch Off?
Switch on a kettle and walk away. When the water boils, the kettle clicks off by itself. There's no thermometer in the water and no computer inside.
How does a kettle know the water has boiled?
The Answer
Steam from the boiling water is led down a narrow tube to a small disc made of two different metals stuck together. The steam heats the disc, the disc bends, and it flicks the switch off.
Most metals get a little bigger when they're heated, but not all by the same amount. Brass, for example, grows more than steel. Bond a strip of each together and warm them, and the strip has to curl, because one side is growing more than the other. In a kettle the strip is shaped as a shallow dome that suddenly snaps the other way once it's hot enough. That snap is the click you hear.
Steam makes a perfect signal. Before the water boils there's very little of it; at the boil there's suddenly plenty, and it's channelled straight to the disc. That's also why a kettle with its lid left open sometimes keeps on boiling: the steam escapes before it reaches the switch.
The first kettle to switch itself off was the Russell Hobbs K1 of 1955. Before that, a forgotten kettle could boil dry. The two-metal strip itself is much older: the clockmaker John Harrison invented it in the 1750s, to keep his sea clocks accurate as the temperature changed.
The principle: Bimetallic strips. Two metals that expand by different amounts are bonded together, so heating makes the strip bend, and the bend can flip a switch at a set temperature.