> The bimetallic component means that one metal heats and expands faster than the other, eventually breaking the circuit.
I thought the circuit that powers the cooking was broken because, when the water has boiled away, heat rises and a magnet holding the circuit closed is weakened by the heat, which allows a spring to pull the magnet away and break the heating circuit. (Magnets are weaker when hot.)
I came to post this video! But I think it's clear that this isn't the design being describe in the original article: that mentions a double-boiler, which is not in evidence here and TC even mentions the double-boiler aspect of earlier designs at around 9:30 in the video. He also says that he was only able to trace the origin of this design back to the '70s.
So it's possible that earlier designs didn't use magnetism, and the magnet-based design was a simplification of earlier water-boiling-at-100c-based designs.
I thought the circuit that powers the cooking was broken because, when the water has boiled away, heat rises and a magnet holding the circuit closed is weakened by the heat, which allows a spring to pull the magnet away and break the heating circuit. (Magnets are weaker when hot.)