10,000 rpm

You know you’re a geek if you find this cool!

homopolar motor

How does this work?

When you touch the wire to the side of the magnet, you complete an electric circuit. Current flows out of the battery, down the screw, sideways through the magnet to the wire, and through the wire to the other end of the battery. The magnetic field from the magnet is oriented through its flat faces, so it is parallel to the magnet’s axis of symmetry. Electric current flows through the magnet (on average) in the direction from the center of the magnet to the edge, so it flows in the radial direction, perpendicular to the magnet’s axis of symmetry. If you took physics at some point, it’s possible that you’ll remember the effect that a magnetic field has on moving electric charges: they experience a force that is perpendicular to both their direction of movement and the magnetic field. Since the field is along the symmetry axis of the magnet and the charges are moving radially outward from that axis, the force is in the tangential direction, and so the magnet begins to spin. Neat!

It’s called a homopolar motor because you never need to reverse the polarity of any motor component during operation, unlike the other types of motors that we’ve described. I first learned about homopolar motors in an article by David Kagan, in the magazine The Physics Teacher, February 2005. It turns out that it’s been around longer than that: it was invented in 1821 by Michael Faraday. Somewhat surprisingly, this is more than just a curiosity: motors of this design are currently being developed for quiet, high-power applications.