How does a kettle know when to switch off?

Early electric kettles came with built-in danger: it was relatively easy to switch them on, go off and do a chore or two, and then forget about them. If you were lucky, when you came back a few minutes later, you'd find your kitchen filled with clouds of steam. If you were unlucky, your kettle element might burn out, blow a fuse, or even start a fire.

Thankfully, virtually all modern kettles switch themselves off automatically using thermostats (mechanical, electrical, or electronic devices that respond to changes in temperature). Many are based on designs developed by English inventor John C. Taylor, whose companies Otter Controls and Strix Ltd have developed more than a billion thermostats of this kind worldwide.

How do they work? The simplest ones are mechanical and use a bimetallic thermostat (described in our main article on thermostats) integrated into the element unit at the bottom of the kettle. It consists of a disc of two different metals bonded tightly together, one of which expands faster than the other as the temperature rises. Normally the thermostat is curved in one direction, but when the hot water reaches boiling point, the steam produced hits the bimetallic thermostat and makes it suddenly snap and flex in the opposite direction, a bit like an umbrella turning inside out in the wind. When the thermostat snaps open, it pushes a lever that trips the circuit, cuts off the electric current, and safely switches off the kettle. More sophisticated kettle thermostats (used in systems such as the fashionable Marco Über coffee boiler) are entirely electronic and allow water to be heated to precise temperatures and maintained there indefinitely by repeatedly switching the current on and off.

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