Lenz's Law Demonstration Using an Ultrasound Position Sensor

One of the very popular demonstrations used in introductory physics courses to illustrate Lenz's law is the “slowly falling magnet.” In its simplest version it requires only a powerful cylindrical magnet and a metal tube, typically of copper or aluminum. When dropped in the tube the magnet takes significantly longer to reach the other end than a geometrically similar but nonmagnetic object. This demonstration has been adapted for use in large classes using a camera to monitor the magnet as it approaches the end of the tube.1 Small versions that can be used for hands-on experiments also have been developed2 or are available commercially.3 This classical demonstration in its various forms almost never fails to impress first-time viewers.