I was asked to try and produce this demonstration of Lenz's law. This law states that an induced electromotive force (emf) always gives rise to a current whose magnetic field opposes the original change in magnetic flux.
The idea of this demonstration is that if a magnet is moved close to the ring on the right hand side of this device, a current will be induced in the ring; this will produce a magnetic field which opposes that produced from the magnet and so the arm will swing as the ring moves away from the moving magnet (like trying to push two magnets together with their north poles facing each other: the magnets repel each other). However, if the same is tried on the ring at the left hand side of the device, little or no reaction will be seen, as the ring is not complete: there is a gap in the bottom so there is no complete circuit and a current cannot flow.
This is the Mark 1a version of the model. In the Mark 1 version I attached the rings to the arm using tiny nuts and bolts - forgetting that the nuts and bolts themselves were magnetic, so they were simply attracted to the nearby magnet. This version uses plastic pins and glue. However, I think I need a Mark 2 version with larger rings and, perhaps, a lighter arm which will hopefully produce a more obvious movement.
Ah, the joys of being a science technician in a school!