Quick Answer: The U-Core Magnetic Field Apparatus is a teaching electromagnet in which current-carrying coils are fitted on a U-shaped iron core, producing north and south poles at the two open ends. It is used to show that an electric current creates a magnetic field and that the field can be switched on, off and reversed.
An Electromagnet You Can Switch and Reverse
A permanent magnet cannot be turned off. The u-core magnetic field apparatus can. When a direct current flows through the coil, the iron core concentrates the field produced by the turns and the two ends of the U become opposite poles; when the current stops, most of the magnetism disappears because a soft iron core holds little of it. Reversing the connections swaps the poles, which students can confirm with a plotting compass.
The U shape brings both poles side by side, so the field between them is strong and fairly uniform, much as with a Horseshoe Magnet. The difference is control: with a rheostat or variable supply in the circuit, the class can increase the current step by step and watch the lifting force grow, then plot a graph of paper clips or iron washers held against current.
Placing an iron bar, called a keeper or armature, across both poles closes the magnetic circuit. The field then passes almost entirely through iron instead of air, and the grip on the bar becomes far stronger than the pull on loose objects at either pole alone. This is the principle behind electric door locks, relays and lifting magnets.
Specifications
| Type | Electromagnet on a U-shaped core |
| Core | Iron, U-shaped, poles at the two open ends |
| Operation | Low-voltage DC through the coil |
| Demonstrates | Magnetic effect of current, pole reversal, lifting force, closed magnetic circuit |
| Rated supply voltage and coil details | Confirm at enquiry |
Applications
- Showing that an electric current produces a magnetic field
- Investigating how lifting force changes with current
- Finding and reversing pole polarity with a plotting compass and the right-hand grip rule
- Mapping the field between the poles with iron filings
- Explaining relays, electric bells and lifting magnets
Care & Handling
- Operate the u-core magnetic field apparatus from a low-voltage DC supply only, and switch off between readings so the coil does not overheat.
- Keep iron filings in a sealed shaker and lay paper over the poles before sprinkling; filings stuck to the core are hard to remove.
- Store it with an iron keeper bar across the poles where one is available, and keep the core dry to prevent rust.
Why Choose LabEquip
Physics teachers buy this apparatus for the magnetic effect of current, where a switchable electromagnet makes the point far better than a permanent magnet. LabEquip lists it in its General Items; students who want to wind their own coils can use DCC copper wire. Enquiries go through the contact page.
Frequently Asked Questions
How is this different from a horseshoe magnet?
Both have their poles side by side at the open ends of a U. A horseshoe magnet is permanent and always on. The U-core apparatus is an electromagnet: it works only while current flows, its strength rises with current, and reversing the current reverses the poles.
How do I find which end is the north pole?
Bring a plotting compass near one pole: the north end of the needle points away from a north pole and towards a south pole. The right-hand grip rule predicts the same result, with fingers following the current and the thumb pointing to north.
Why does an iron keeper across the poles hold so strongly?
The keeper completes a loop of iron, so the magnetic field no longer has to cross an air gap. Iron carries the field far more easily than air, so the force on the keeper becomes much larger than the pull on objects at a single pole.
Why does it still hold a little after the current is switched off?
The iron keeps a small amount of residual magnetism. Soft iron keeps very little, which is why it is used in electromagnets, but a keeper resting across the poles may cling until it is pulled or tapped free.
What power supply should be used?
A low-voltage DC supply, such as a battery or laboratory power pack, within the coil’s rated voltage, which should be confirmed when you enquire. Include a switch and, ideally, a rheostat to adjust current. Never connect the coils to mains.
How can the magnetic field pattern be shown?
Place a sheet of card or paper over the poles and sprinkle iron filings thinly, then tap the card gently. The filings line up along field lines, showing a strong concentrated field between the two poles.
Last Updated: September 2026
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