What it shows:
A magnet tethered over a spinning aluminum disc levitates due to induced currents in the disc.
How it works:
As the disc spins, electrical currents are induced in the aluminum as it moves with respect to the magnet. These induced currents create a magnetic field which, in accordance with Lenz's law, opposes the field of the magnet. The magnetic repulsion causes the rider to levitate about 1cm above the disc. Lenz's law also says that the induced field will oppose the motion that causes it. The magnet therefore tugs at the disc trying to slow it and, in the process, is pulled along in the direction of rotation of the disc.
figure 1. Looking down onto the disc: magnetic rider and safety screen
The disc is made of 6mm aluminum sheet, and is 70cm in diameter. There is a clearance of 0.5cm between it and the plywood casing. The aluminum disc is mounted upon a 25cm diameter 6mm brass plate which is attached to the drive shaft. A 4mm clear Plexiglas safety cover overlays the top of the disc and casing.
The magnetic rider consists of a pair of samarium cobalt magnets taped to an aluminum sheet (10×10cm). This sheet is tethered by cord to a wooden anchor block beside the disc (figure 1). A Plexiglas safety screen (8cm high) crosses the disc in case the rider comes free.
The drive mechanism is contained within the plywood casing below the disc (figure 2). The motor is a General Electric 1/4 hp 1725 rpm AC motor connected to the drive shaft by a drive belt with a gear ratio of 1:2.5. The drive shaft is secured by two sets of bearings attached to an aluminum U beam bolted to the walls of the demo casing.
figure 2. Drive mechanism for the rotating disc.
Setting it up:
The demo is mounted on wheels and only needs to be rolled in and plugged in (wish they were all that easy). It should be oriented such that the Plexiglas shield protects the audience.
A clear demonstration of magnetic levitation devices that are coming into increasing commercial use. You can also demonstrate magnetic breaking by pressing the magnet rider down onto the disc after the motor is switched off (don't do this with the motor on - it might burn out). This shows the opposition to motion idea very well, but isn't good for the turntable.