A simple but effective device for demonstrating Lenz's Law using eddy currents induced in the wall of a copper tube by a falling magnet. The demonstrator consists of a 12" long copper tube and two apparently identical metal slugs with rubber end caps. One slug is plain steel and falls through the copper tube quickly. The other slug is a strong neodymium magnet that falls very slowly through the tube due to the braking effect of eddy currents. Includes activity guide.