5K20.30-Jumping ring


Video:  Watch this demo
MIT demo video

Description: This demo shows how an aluminum ring can float around a vertical inductor.
If enough current is applied, the ring will “jump” off the vertical inductor due to opposing magnetic fields.


  • VariAC
  • 250 Turn coil
  • Large/Thick ring
  • (Optional) Ring with an open slit in it
  • Three iron inductors
  • VariAC to banana plug cable
  • (Optional) LN2 with bowl and protective gear

Setup Procedure:

  1. Try to find a good spot so that nothing breakable will be above it.
  2. Insert the three inductors and connect the wires.
  3. Test to make sure the VariAC is working.
  4. Include a spare fuse or two.

Demonstration Procedure:

  1. (Optional) Place the ring with the open slit on the apparatus and turn on the VariAC.
  2. (Optional) Note that nothing happens. Turn off the VariAC and remove the open ring.
  3. Place the closed ring on.
  4. With the VariAC set to a low voltage, turn it on. By varying the voltage you can make the ring levitate.
  5. When you are ready to jump the ring, turn the VariAC off. Set the voltage up to high.
  6. (Optional) Place the ring into LN2 for a few seconds to super cool it. This makes the ring jump more effectively.
  7. Turn on the VariAC and then turn it off very quickly (no more then a few tenths of a second or it will blow the fuse)!

Concept: This demo is used to show how magnetic inductance works. The magnetic field produced by the solenoid will induce a current in the iron inductor, which will then produce its own magnetic field. The magnetic field produced by the iron will induce a third current in the ring, which will make the ring produce a magnetic field opposite to that of the iron. The opposing magnetic fields will repel each other, pushing upwards on the ring. The reason that no force acts on the open ring is because current cannot flow through an open circuit, so the iron will not induce a current in the open ring (resulting in no force on the ring).

Note: When jumping the ring, there is a chance the ring will ricochet off something and hit a student.
Use caution and good judgement when doing this demonstration.