The kettle uses steam as a signal
The switch is not waiting for a timer or measuring the water’s temperature directly. In many electric kettles, a narrow passage carries steam from the body of the kettle toward a temperature-sensitive mechanism in the handle or base. While the water is heating, the signal stays relatively cool; once vigorous boiling begins, hot steam reaches the mechanism.
That makes steam a useful marker for the moment the water has reached its boiling point. The kettle can detect the change without placing its electrical switch in the water.
Why the switch snaps instead of easing off
The sensing element is a bimetallic strip: two bonded metals that expand by different amounts when heated. Because they are joined, they cannot simply grow independently, so the strip bends. When steam heats it enough, the strip reaches a threshold and suddenly flips shape, pushing a mechanism that separates the electrical contacts and cuts power to the heating element.
The sudden movement matters. A gradual bend could leave the contacts barely touching, allowing the heater to keep cycling or arc. The snap action gives the switch a decisive change from on to off.
A small passage controls a large amount of heat
The steam route and the strip together turn a physical change in the kettle into a mechanical command. If the passage is blocked or the sensing parts are damaged, the kettle may not shut off as intended, which is why an automatic cutoff is a control mechanism rather than a substitute for ordinary care. In normal operation, a little stream of vapor is enough to make the kettle’s much larger heating element go quiet.
