DESCRIPTION: Place a metal ring over the core of an induction coil.  Turn on the AC power to the coil.  The ring will jump high in the air.

The changing magnetic field in the coil due to the change in current in the coil induces a current in the metal ring.   Lenz's Law states that the magnetic field of the ring opposes the magnetic field of the coil and forces the the ring to fly away.  Cooling the ring in liquid nitrogen will get a really great jump, but be careful that the ring does not ricochet off the ceiling. 

A second ring with a slit cannot have an induced current and is unaffected by the magnetic field.  This device is commonly named the Thomson's coil.

Phys. Teach., Vol. 35, No. 2, February 1997 Pages 80 - 83, Forces on the jumping ring
Harvey E. White and Hans Weltin, Electromagnetic Levitator, AJP 31, 925-929 (1963).
E. J. Churchill and J. D. Noble, A Demonstration of Lenz' Law?, AJP 39, 285-287, (1971).
W. R. Towler and J. W. Beams, Magnetic suspension for lecture and classroom demonstrations, AJP 44, 478-480 (1976).
Arthur R. Quinton, The AC Repulsion Demonstration of Elihu Thomson, TPT 17, 40-41, (1979).
S. Y. Mak and K. Young, Floating metal ring in an alternating magnetic field, AJP 54, 808-811 (1986).
Thomas D. Rossing and John R. Hull, Magnetic Levitation, TPT 29, 552-562 (1991).
Richard V. Mancuso, Letter: Jumping Ring Referenced, TPT 30, 196 (1992)
Gary Stix, AIR TRAINS, Scientific American, August 1992, pp. 102-113.
Ron D. Edge, Corrections to Levitation Paper, TPT 34, 329 (1996).
David Simmons and Robert R. Speers, Magnetic Damping of a Mass-Spring vertical Accelerometer, TPT 35, 49-50 (1997).
Jonathan Hall, Forces on the Jumping Ring, TPT 35, 80-83 (1997)
Kenneth E. Jesse, Measuring Current in a Jumping Ring, TPT 35, 198-199 (1997)
Paul Tanner, Jeff Loebach, James Cook, and H. D. Hallen, A pulsed jumping ring apparatus for demonstration of Lenz's law, AJP 69, 911-916 (2001).
Paul J. H. Tjossem and Victor Cornejo, Measurements and mechanisms of Thomson's jumping ring, AJP 68, 238 (2000)
Aaron McAlexander, Apparatus for Teaching Physics: PSSC Turbo Ring Flinger, TPT 43, 613 (2005)

EQUIPMENT: Home made Jumping Ring apparatus, also known as the Thompson's Coil. 
The construction plans are here: 

SETUP NOTES: Try it before classroom use to check that it does not blow the fuse on the mains power line.