This motor consists of four simple parts: a cylindrical "neodymium" (NdFeB) magnet,¹ a steel nail, a D-cell battery, and a short piece of (nonferromagnetic) wire. It's amazingly easy to put together and runs beautifully every time. Just assemble the pieces as shown in Fig. 1 (the magnet sticks to the head of the nail and the tip of the magnetized nail is attracted to the ferromagnetic bottom of the battery). Now, hold one end of the wire to the top of the battery and touch the other end to the side of the magnet. A current flows through (and/or along the surface of) the magnet as shown in Fig. 2. The charge carriers moving in a magnetic field experience a force whose direction is given by the right-hand rule. This direction defines the sense of rotation of the motor. The device is called a homopolar motor.²

Figure 1.

Figure 2.

Column Editor's Note: This "Gem" was inspired by a demonstration described in a booklet written by "The Stray Cats," a group of Japanese physics teachers.