Lenz’s Principle Verification Apparatus

Quick Answer: The Lenz’s Principle Verification Apparatus shows that an induced current always opposes the change that produces it. The listing image shows a solid aluminium ring, a slotted aluminium ring and an insulating stand with a pivot; a bar magnet pushed into the solid ring makes it move away, while the slotted ring does not respond.

Two Rings, One Magnet

In the usual arrangement for this Lenz’s law apparatus, the two aluminium rings sit at opposite ends of a light beam balanced on a pivot at the top of the stand, free to swing in a horizontal plane. When a bar magnet is thrust into the closed ring, the magnetic flux through the ring increases, a current is induced around it, and that current makes the ring behave like a magnet whose near pole repels the approaching one. The ring moves away and the beam turns.

Pulling the magnet back out reverses everything. The flux now decreases, the induced current flows the other way, and the ring follows the retreating magnet. The slotted ring is the control: its gap breaks the circuit, so no current can circulate and that side of the beam stays still when the magnet approaches. Aluminium is used because it conducts well, is light and is not attracted to a magnet on its own, so any movement must come from induction.

Lenz’s law is really a statement of energy conservation. If the induced current helped the change instead of opposing it, the ring would pull the magnet in faster and faster, creating energy from nothing. Whether a bar magnet is included should be confirmed when ordering.

Specifications

Applications

• Verifying Lenz’s law and predicting the direction of induced current
• Linking electromagnetic induction to conservation of energy
• Explaining magnetic braking and eddy current dampers
• Comparing a closed conductor with a broken circuit in one demonstration

Care & Handling

• Move the magnet smoothly along the ring’s axis and avoid striking the ring, which can knock the beam off its pivot.
• Let the beam come to rest before each trial so earlier swinging is not mistaken for induction.
• Keep the apparatus away from draughts and fans, which can turn a light beam on their own.
• Store the magnet with a keeper or away from the stand, and never drop it, as knocks weaken magnets.

Why Choose LabEquip

Senior secondary and college physics departments buy this apparatus because it gives a clear yes-or-no result for Lenz’s law in a few seconds. LabEquip offers it through the STEM kits range, and the Eddy Current Disc Demonstration Model extends the same opposition idea to a spinning conductor. Reach us through the contact page.

Frequently Asked Questions

Why does the closed ring move away when the magnet approaches?

The approaching magnet increases the magnetic flux through the ring, so a current is induced in it. By Lenz’s law that current flows in the direction that opposes the increase, making the ring’s face act like a pole of the same kind as the approaching one. Like poles repel, so the ring is pushed away.

Why does the slotted ring not move?

The slot breaks the conducting loop, so no current can flow around the ring even though an EMF is induced across the gap. With no current there is no magnetic effect to oppose the magnet, and the ring stays where it is.

Does it matter which pole of the magnet is used?

No. Either pole pushed in repels the closed ring, and either pole pulled out attracts it. What changes is the direction of the induced current, which reverses when the other pole is used, so the ring still opposes the change.

What happens if the magnet is held still inside the ring?

Nothing. Induction depends on a changing flux, so a stationary magnet induces no current and the ring feels no force. Movement starts again only when the magnet is moved in or out.

Why are the rings made of aluminium rather than iron?

Iron would be attracted by the magnet whether or not any current flowed, which would hide the effect. Aluminium is non-magnetic, light and a good conductor, so the only force on it comes from the induced current.

How does Lenz’s law relate to conservation of energy?

The opposing force means work must be done to move the magnet, and that work becomes electrical energy in the ring and then heat. If the induced current assisted the motion, energy would appear from nowhere, which is impossible.

Last Updated: September 2026