very simple screw 'motor'

screw motor

The simple 'motor' made from a magnetised screw and battery

How to make a spinning screw:
1) put a small but powerfull (e.g. NdFeB) magnet onto the head of a magnetisable screw. This will make the other end (the pointed bit) of the screw magnetic and so be attracted to the bottom (- say) of an AA battery.

2) the screw-magnet needs to be able to hang self supporting.

3) connect a copper wire to the other battery electrode (+) and just touch the other end of the wire onto the dangling magnet (or shaft of the nail near to the magnet if you find the magnet has a coating and will not conduct electricity well).

3) the screw will spin round !

Its not immediatly obvious how this motor works beacuse the magnet is hanging pole down (up) rather than mounted side-ways. As it spins the magnetic field will not change very much (it does spin but does not sweep out a changing magnetic field). I guess that when the wire makes contact with the magnet the current produced by the simple circuit at the junction will be in the direction towards / or away (depending on the battery connections) from the center of the magnet - which is at right angles to the magnetic field. The resultant force will be at right angles to both the current and magnetic field and will tend to cause the magnet to spin. As there is so little friction it picks-up speed even if the forces are quite small or if there is an intermittent / poor contact.

We have a low resistance circuit so a typical AA battery will produce a decent current (a few 100's mA), we have a powerfull magnet and, most importantly, we have a wonderful low-friction bearing produced by the hanging screw point. Any slight change in field brought about by the magnet spinning (wobbling) is enough to get the thing moving and then each time the wire touches, it adds a bit more energy.

Timo, a friend of mine told me about this little motor (he actually demonstrated it to me in just a few seconds) and told me that he found the information on a German web site. When I get the URL of the original article I will add it to this page.


Dr Jonathan Hare, Physics Dept., The University of Sussex
Brighton, East Sussex. BN1 9QJ.

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