Speedometer

It occurred to me today that I don't actually know how a speedometer works. I looked at this: https://www.explainthatstuff.com/how-speedometer-works.html Whilst mine is certain to be a modern electronic one I really liked the old electromagnetic induction idea and will try to get it into my teaching. The engine turns and pulls a magnet round with it. The magnet passes a metal cup and induces a current in the cup. This turns the cup into a kind of electromagnet. By Lenz's Law, this new electromagnet must have poles that try to oppose the original turning of the magnet. In other words, the poles are attracted to the original turning magnet, trying to drag it back. This means that the cup gets pulled round. The cup is actually attached to a rotary spring to stop it moving and the bigger the current the bigger the force so the further the spring turns. The spring is attached to a needle which is what you see.