Ring Flinger Lenz's Law

electromagnetic induction - Lenz's Law - magnetic levitation

What it shows:
Changing magnetic flux induces a current in a metal ring; the resulting magnetic field opposes the primary field, repelling the ring and flinging it into the air.

How it works:
The solenoid sits vertically with the iron core - a 60cm long cluster of 1mm iron rods - resting inside it and protruding 42cm. The solenoid is a 400 turn coil with an air core inductance of 4.7mH, wound with 14 AWG wire insulated for 110V. The coil is connected to 110V AC.

The rings are 4mm aluminum, 2cm deep and 4.2cm inner diameter so that they fit loosely over the core. Place the ring on top of the solenoid and turn on the power; the ring will shoot off the end of the core to a height of about 2m. Alternatively, turn on the power and place the ring over the core, and it will hover about half way up. A second aluminum ring is identical save for a split, which prevents a circuital current being set up so no field can be generated; the ring just sits there. Also available is a 50 turn (20AWG) wire loop connected to an 18V bulb. The induced EMF lights the bulb when the ring is lowered over the iron core; its brightness will increase dramatically as you move the ring down (see Comments).

Figure 1. The Ring Flinger

Setting it up:
Sits on lecture bench. Make sure iron core is in place before turning on the supply; the low inductance will blow a fuse.

Comments:
For a much more spectacular fling, dip the ring into liquid nitrogen. This is explained in the Jumping Ring demo. Take care when using the coil and bulb, as it can easily burn out if dropped to the base of the rod, due to the very large flux down there.

For a bit of showmanship and sleight-of-hand, place the split ring over the core and ask a volunteer to hold it down for five seconds (no problem of course). Slyly replace the split ring with the complete ring, and wager a second volunteer they cannot hold it down for ten seconds. This is essentially a 400 to 1 step down transformer, and with an induced current of about [+ how much?], the ring is unbearably hot in seconds! Rating ****

Reference:
C. Schneider and J. Ertel have published a very detailed explanation of how the jumping ring "really works": Am J Phys 66(8), pp 686-692 (1998). Many good references are also given. Apparently the jumping ring was first demonstrated by Elihu Thomson in 1987 to the American Institute of Electrical Engineers to help promote the supremacy of his ac dynamos over Edison's dc system.