Magnet-nail Motor The photograph on this page shows a "motor" consisting of a spinning nail and magnet. I learned of a device such as this in a phone conversation with a colleague who had seen one demonstrated.

Parts List AA battery Common nail Neodymium magnet Wire Procedure Place the nail head on the magnet and centre it Attach the nail tip to the bottom of the battery Hold one end of the wire to the top of the battery "Brush" the other end of the wire to the outer edge of the  magnet Observations The magnet and nail begin spinning (voila, the motor).   The rotation rate can be quite high...I'm guessing in the  tens of cycles per second. My colleague said that in the demo he saw that the lower wire "connection" could be almost anywhere along the nail shaft. I could not get this to produce a reliable rotation.

Other comments

This motor runs with no appreciable sound and remains spinning for quite some time after the connection is broken (the only contact is between the nail point and the bottom of the battery). I'm finding it difficult to keep the nail/magnet combination from spinning wildly. My battery doesn't have a dimple on the bottom so the nail wanders around once it starts spinning. I don't have a smaller magnet at the moment. I suspect a smaller diameter magnet would be more stable. I must admit that I didn't expect the battery case to be ferromagnetic. The three I tested (Maxell, Duracell, Energizer) were and so could be used for this demonstration. 

Follow up

According to a colleague, this type of motor is described in The Physics Teacher, December 2004, page 553 and a modified version appears in The Physics Teacher, February 2005, page 68.

I am continuing to experiment with the motor. Recently I found that I could keep the motor more stable by hanging the nail from the positive terminal rather than from the wider negative one. Using this approach I began measuring the "spin time" after pulling the lead from the battery. In one such test the motor kept spinning for an amazing (well, to me at least) 88 s!

Follow up (September 10 2005)

Thanks to a colleague, Ernie Quan, I now have a much improved motor. He gave me a smaller magnet that he obtained at Lee Valley Tools. This new arrangement spins so effectively that the nail and magnet usually fly off the battery. I will post a photograph of this new setup in due course.

Follow up (September 12 2005)

I should have noted some other observations in earlier postings. I deliberately chose a piece of lamp cord for my wire "brush." Some have written me to say they can't get their version of the motor to work. Although I haven't yet measured a current it is "large." If I let the motor run for say ten seconds the battery becomes quite warm.

The question of how the back emf might be generated for this motor has been raised. Well, let's put it this way: it wasn't raised by me and I don't have an explanation as yet. Besides, in the present configuration the nail just spins faster and faster until it comes flying off.

Let's see what type of ads Google conjures up for this page...

Peter Vogel
Vancouver BC

Back to Physics On-line