UC Santa Cruz Physics Demo room

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Lenz’s Law – Jumping Ring

Figure 1
Figure 1
Figure 2
Figure 2
Figure 3
Figure 3

A coil with an iron core and two aluminum rings is used to demonstrate electromagnetic induction and Lenz’s Law. One of the rings has a slit cut in it, and the other does not (see figure 3). The rings are placed around the core. When the apparatus is turned on, the solid ring is ejected into the air. The ring with the slit remains. The effect is enhanced if the ring is cooled with liquid nitrogen.

If the button is held down and the ring is dropped from the top of the iron bar, it will remain suspended as shown in figure 1.

The apparatus also includes a light bulb attached to a small copper coil which shines when the coil is placed around the core and the button is pressed.


  • Iron core solenoid with push button(see figure 1)
  • Solid aluminum ring
  • Aluminum ring with slit cut in it(see figure 3)
  • Small copper coil with light bulb attached
  • Liquid nitrogen for cooling(optional)


Place the two aluminum rings around the iron core. Plug the apparatus in and press the button to send current through the coil. The solid ring will eject into the air, while the slitted ring will remain unaffected.

Now remove the rings. Press and hold the button and drop the solid ring from the top of the iron bar. Notice that the ring becomes suspended at an equilibrium position until we release the button, shutting off current flow.

Remove the solid ring again. Place the copper coil with the attached light bulb around the iron bar. Press the button and observe the light bulb shine while we hold the button down.


When we turn our solenoid on, the sudden change of a magnetic field induces a current in our aluminum ring that flows in the opposite direction as the coil in the solenoid.  These opposite currents repel one another, ejecting the aluminum ring into the air.

Why does the solid ring shoot up, but not the slitted ring? The slitted ring does not eject into the air because an induced current cannot complete a loop around the ring.

Why does the solid ring stay suspended at the same position each time?

Why does the light bulb shine? A changing magnetic flux induces a voltage in the coil, creating current and causing the light bulb to shine.


The use of liquid nitrogen is not needed to produce significant effects, however can be used to make predictions and compare the effects at room temperature versus when cooled.