Jumping Rings
This demonstration shows, quite dramatically, the induction of Eddy currents. It consists of an strong electromagnet, whose iron core sticks a few inches out of the coil, and a set of metallicrings that fit around the iron core.The coil connects directly to an electrical outlet and the alternating current produces an alternating magnetic flux trough the metallic rings. The sense of the induced current in the ring is governed by Lens' law resulting in the ring behaving like a magnet with the opposite polarity to that of the iron core of the coil. The repulsion between these magnets propels the ring into the air.
The magnitude of the induced current can be increased by reducing the electrical resistance of the rings, which can be accomplished by cooling down the ring. Using liquid nitrogen for this purpose is veryeffective.