Lenz's Law

Lenz's law, named after the physicist Heinrich Friedrich Emil Lenz who formulated it in 1834, states that the direction of the current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes the initial changing magnetic field. Or as informally, yet concisely summarised by D.J. Griffiths: 'Nature abhors a change in flux'. [Wikipedia]

In this video, Tim demonstrates a variety of Lenz's Law demonstrations. At its simplest, you can just drop a magnet down a metal tube, maybe made of copper or aluminium, and be surprised at how slowly it descends, despite the fact that copper and aluminium are not attracted to magnets.

Very recently Tim came across an unusual demonstration, involving a large block of aluminium with a slot cut along its length. Dropping a spherical neodymium magnet down the length of the slot produces some interesting and unexpected effects, depending on the orientation of the magnet at the point at which you let it drop.