Boiling kettle

[gav9000]

How does a kettle know when to switch off upon boiling ? Someone told me they have a mercury tilt switch, but I think he may have been watching too many action movies.

[MORELLO]

Take the lid off and it will take longer. It's to do with the pressure created by the steam, which triggers the switch. You'll notice it doesn't turn off on boiling, rather than a few seconds later when the pressure has built up.

[New Forester]

There is a vent for the steam which has a heat-sensitive switch -- usually in the top part of the handle.  Before it boils, the small amount of steam is not enough to vent by this route -- when it's boiling, there is lots of gas and it has to go somewhere.

 

If you leave the lid off, the steam is not forced over the switch and it will carry on boiling.  If the kettle is dry, there will be no steam.  There is then a second switch near the element which will trip it out to stop the whole thing melting.

 

Each switch is a bimetallic one.  Two metals of different expansion rates are joined, each forming one side of a metal strip.  As the temperature rises, the strip is forced to bend, eventually closing the electrical contact (or, as in a fridge, opening it).

[MORELLO]

...like I said, New Forester...

[rekstout]

In the interests of science, I have smashed apart a broken kettle in our office and can verify that there is a small bimetallic switch which opens the circuit and disconnects the heating element - it is not a pressure switch was first suggested.

[gav9000]

Thank you all, a most impressive and coherent set of answers - all very useful.