Mobile phones are made to vibrate by a very small electric motor with an eccentrically mounted (off-centre) weight on the shaft (click here to see one). When the motor spins, this unbalanced weight makes the phone vibrate in exactly the same way that a solitary soggy duvet in a washing machine makes it shake, rattle and roll all over the kitchen.
The motors that are used in mobile phones really are very tiny: some of them aren’t much bigger than 4 mm across and maybe 10 mm long, with a shaft well under 1 mm in diameter. It wasn’t very long ago that these titchy motors were regarded as a mechanical marvel with a price tag to suit. Now we can make then by the million, and cheaply enough to use them in things like throw-away vibrating toothbrushes that sell for a fiver.
There are a variety of videos on YouTube, showing what these motors look like when you take them out of their phone or vibrating toothbrush.
Here’s a demo of a ringing mobile phone with the motor exposed:
Answer by Richard Ellam
Image source: Matthew, on Flickr