We have aleady shown the working of the Homopolar Motor. A subsequent development of this motor was designed by Norihiro Sugimoto and Hideo Kawada, which involves sticking a neodymium magnet on both ends of an AA battery, and then placing this on a conducting surface. In order to get both torques to act in the correct direction so that rolling will occur, it is necessary to orientate each end magnet so that like poles face one another.
There are two problems with this motor. One is the difficulty in getting a good electrical contact between the magnets and the surface. The other is that eddy currents induced within the conducting surface itself acted as magnetic braking. See Lenz's Law Demo.
An interesting development in the rolling mechanism has been made by Seán Stewart, from the Petroleum Institute in Abu Dhabi.
By using a piece of copper wire shaped into a U, as shown in the photograph above, there is no longer any need for the surface to be conducting, and also the magnetic braking effect is avoided.
This Homopolar Roller will roll across a wooden desk or along the floor at a brisk rate, as you can see in the video clip.
We gratefully acknowledge the help of Seán Stewart with this article. You can download his full paper here.