The basic operation of a Teasmade is a sealed kettle with a tube passing from the bottom of the kettle out into the tea pot. As the water heats nothing much happens. The water will expand but that will be taken up by the air in the kettle. However, when the kettle boils the water is converted to steam which generates an enormous pressure (like a steam train!) and forces the water out and into the pot. It is quite violent so the tea gets a good stir!
There are two cut-out switches in most tea makers. One ensures that the pot is present so that you can’t boil water out into the open. The other is a thermostat bolted with good thermal contact to the actual element.
The thermostat works by having a fixed contact and a sprung contact made of a bi-metallic material. As the element heats up the bi-metallic contact strip gradually bends away from the fixed contact until at a certain temperature it opens the circuit – hopefully slightly above boiling point! – and cuts off the kettle. The bi-metallic element consists of two different metals. Thin strips of the two metals are bonded together – like a thin strip of plywood. Each metal has a different coefficient of expansion, so as the combination heats up, one of the strips will expand more than the other and the combined strip will bend. A suitable contact is welded to this strip in such a way as to either MAKE when the desired temperature is reached (i.e. an alarm) or to BREAK when the temperature is reached (i.e. a cut-out, as in our kettle).
Thank you to reader our Dave for this description.