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Thompson's (jumping) Rings


An iron-core solenoid with two switches- a momentary switch and a toggle switch- is mounted inside a plexiglass enclosure and has a power cord to connect to mains power. The top has a post lined up with the axis of the solenoid so that conducting rings may be stably levitated over it.


This iron core solenoid is a great demonstration of Lenz's Law. By using AC straight from the wall, it demonstrates how induced current from a magnetic field depends on a change in magnetic flux, and the resulting Lorentz force from these currents causes the rings to levitate!

There are several rings available, various aluminum rings, including one with a slit cut in it to demonstrate that a flowing current is required, a brass ring, and a light bulb connected to many coils that lights up when placed over the solenoid. For the ring with the slit, one can demonstrate that the floating is due to an inductive and not a resistive effect. Connecting alligator clips across the slit will do nothing, but inserting a aluminum sheet into the slit will restore some of the ring's jumping properties (of course there is still a finite air gap between the ring and the inserted sheet metal).


  • Jumping ring apparatus
  • Aluminum ring
  • Aluminum ring with slit
  • Copper solenoid with lightbulb


Simply set the device to “float” mode, and place a ring around the core, and it will float. If the device is set to “jump,” then the solenoid will receive a short pulse of electricity when the momentary switch is pressed, and since that translates to an enormous change in flux, the ring jumps a couple feet into the air. Be prepared to catch it!

When the device is in “float” mode1), the copper solenoid may be placed onto the apparatus. As it gets closer to the solenoid it'll light up, getting brighter and brighter and illustrating, in another way, that current is flowing.


Demo room information

Location Q4
Maker Unknown
Current State Working
Using the “jump” mode while the solenoid is on the apparatus may damage the light bulb.
demonstrations/5_electricity_and_magnetism/5k_electromagnetic_induction/thompsons_jumping_rings/start.txt · Last modified: 2020/03/11 15:47 by demoroom