I was making a cup of tea this morning and a thought occurred to me. How does the kettle know that the water has boiled?
It comes down to two players. Steel and Copper. Sometimes they use brass instead of copper, but let’s not make this more complicated than it needs to be.
These metals each have a different ‘co-efficient of expansion’. That basically means that each of the metals expand a different amount for every 1 degree rise in temperature. So, if two strips of these metals cosy up together so one is on top of the other and then you heat them, if one expands more than the other, the whole thing will bend.
It goes something like this:
By Patrick87 (Translation of Bimetallstreifen.svg) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
The fancy term for this device is the ‘bi-metallic strip thermostat’. It is connected to the electrical supply and more often than not is placed in the handle of the kettle. When the metal is at room temperature it stays straight and keeps the electrical circuit closed. Once the water starts boiling, steam is produced and this comes into the handle and heats the metal, which we now know will bend, thus breaking the electrical circuit when you hear the *click*.
Et voila, your kettle turns off. Simples.
That does mean that if you leave the lid open, there may not be enough steam flowing past your smarty-pants strip so the kettle may not be deactivated. Try it.
How it all came about in the first place
The earliest kettles were rather primitive cast iron designs that simply worked by being suspended over an open fire. It stayed that way until the 18th century at which point drinking tea and coffee became a fashionable thing to do.
By Kate Greenaway (1846-1901)
This quickly sparked the beginning of the development of kettles. The first electric kettle was made in 1909 by the Germans. It was met with some resistance due to the danger of mixing water and electricity.
After all, you don’t want to end up looking like this:
Copyright © Tatiana Khlopkova
This led to future designs being really focused on the safety aspect, with everyone trying to think up ways for the kettle to turn itself off so as not to burn the house down. It was Bill Russell and Peter Hobbs who changed the industry in 1955 with their “ground-breaking vapour controlled K1 design”. Wowaweewa.
Never thought kettles could sound so, uh, cool.
Cup of tea anyone?
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