That last answer is so stupid. It doesn't 'detect how much water is in the kettle'! Try turning your kettle on with no water in it at all - it won't magically shut off.Most modern kettles use a steam based cutoff switches. I have just taken my kettle apart (as it's old and isn't working properly) and it has a thermostat (metal plate resting against a switch) that turns the kettle off. So when the water is boiling you get a lot of steam that comes down a pipe inside the kettle chamber. This steam washes over the metal plate, heats it up and causes it to deform turning the switch off.
Sorry for a lot of technicalities -- an electric kettle having an unrestricted spout opening is provided with an electric heating element controlled by a thermostatic simmer control or boil cutout, the thermostatic element of which is located exteriorly of the kettle body above the top thereof and adjacent to the spout opening. A steam diverter scoop extends into the spout opening to intercept the uppermost portion of the live steam flowing toward the spout opening and diverts it into heating relation with the thermostatic element to provide rapid heating thereof upon commencement of active boiling in the kettle. The thermostatic element is housed within the forepart of the kettle handle, which also serves to conceal the wires connecting the thermostatic element to the heating element and other electrical components located in the bottom of the kettle. The diverter includes a shield portion in radiation shielding relation to the thermostatic element to limit radiation of heat to the element from the kettle body. The provision of a thermostatic element which is normally shielded from the rise in temperature in the kettle interior while being exposed a portion of the stream of steam generated upon free boiling within the kettle permits use of a coarsely calibrated, not unduly sensitive, thermostatic element ensuring switching off or other control function of the kettle regardless of altitude. That's it!
Sorry, Kingsrd, but 95% of kettle works on a simple thermostat. And even if it looks they should not work, they can work correctly, if heater is at the bottom of the kettle. Why? Because when water boils, water keeps it's boiling point, but steam temperature can rise. When steam bubbles are produced on heating element, heat conduction rapidly drops (that's why ice cube or water drop is "dancing" on a very hot surface instead of just quickly vaporize). When heat transfer drops rapidly, and power supplied to the heating element is constant, the temperature of heating element rises rapidly. Thous, if thermostat is set to 102 Celcius degree, it will switch off even in mountains, when boiling temperature is lower.
