Physics demonstrations: Faraday disk
By turning a hand crank, one rotates a copper disk between a pair of magnets (the black disks), one generates an electrical current that runs from the outer edge of the disk to the central axis. Wires connected to these two points runs to the red and black plugs, through which one can measure the voltage difference generated. It isn’t a spectacular amount — I measured about 5 millivolts, max — but it demonstrates the phenomenon known as rotational electromotive force.