Horseshoe Magnet

Quick Answer: The Horseshoe Magnet is a U-shaped permanent magnet whose north and south poles sit side by side at its open ends. Bending a magnet into this shape concentrates a strong, fairly even field in the gap between the poles, which is why it is used for the motor effect, electromagnetic induction and field-mapping experiments.

Why the U Shape Matters

A straight bar magnet has its poles at opposite ends, so the field spreads out widely and weakens quickly with distance. Bending the bar round until the poles face each other across a small gap concentrates the field in that gap. Anything placed there, a current-carrying wire, a moving conductor or a sheet of iron filings, experiences a strong field running from one pole straight across to the other.

That gap is where the key experiments of electromagnetism happen. A wire carrying a current through it is pushed sideways, the motor effect, in a direction given by Fleming’s left-hand rule. A wire moved up and down through the same gap, connected to a galvanometer, produces a current, showing electromagnetic induction. On a sonometer, alternating current in a wire held between the poles sets the wire vibrating at the mains frequency, the alternative to using an Electromagnet for Sonometer.

Younger classes use the same U-shaped magnet more simply: picking up paper clips, sorting magnetic from non-magnetic objects, and showing that the attraction passes through paper, plastic and water.

Applications

• Motor effect demonstrations with a current-carrying wire or aluminium rod
• Electromagnetic induction with a moving wire or coil and a galvanometer
• Mapping the field between the poles with iron filings or plotting compasses
• Determining mains frequency on a sonometer by passing AC through the wire
• Sorting materials into magnetic and non-magnetic in primary science

Specifications

Care & Handling

• Store the magnet with a soft-iron keeper across the poles, or in pairs with opposite poles together, to preserve its magnetism.
• Do not drop, hammer or heat it; shocks and heat scramble the magnetic domains and weaken it.
• Keep it away from phones, bank cards, watches and computer drives.
• Wipe off iron filings with a cloth or tape before storing.

Why Choose LabEquip

Physics teachers keep U-shaped magnets for every bench because so many electromagnetism practicals need one. LabEquip lists this magnet in its General Items, beside the U-Core Magnetic Field Apparatus for comparing permanent and electromagnets. Send bench quantities through the contact page.

Frequently Asked Questions

Why use a horseshoe magnet rather than a bar magnet?

Its two poles face each other across a small gap, so the field there is strong and fairly uniform. That makes it the natural choice when a wire or coil has to sit in a strong field, as in the motor effect and induction experiments.

How do I tell which pole is which?

Most magnets are colour-coded, but you can check with a compass: the north-seeking end of the needle is attracted to the magnet’s south pole and repelled by its north pole. Repulsion is the reliable test, since attraction also happens with unmagnetised iron.

What is the keeper for?

A keeper is a soft-iron bar placed across the poles during storage. It gives the field a closed path through iron, which helps the magnet retain its strength over years instead of slowly weakening.

How do I demonstrate the motor effect?

Place a light wire or aluminium rod between the poles, resting on two rails, and pass a current through it from a low-voltage supply. The rod rolls sideways; reversing the current or the magnet reverses the direction, as Fleming’s left-hand rule predicts.

What makes a permanent magnet lose strength?

Dropping or hammering it, heating it strongly, storing it without a keeper and placing like poles together all disturb the alignment of its magnetic domains. A weakened magnet can often be remagnetised with a strong electromagnet.

Can it be used in the sonometer experiment?

Yes. Pass low-voltage AC through a sonometer wire of any metal while the magnet straddles it. The force on the current alternates each cycle, so the wire vibrates at the supply frequency and can be tuned to resonance with the bridges.

Last Updated: September 2026