Demagnetizing and Magnetizing Coil Apparatus

Quick Answer: The Demagnetizing and Magnetizing Coil Apparatus is a long solenoid mounted on a board with supply terminals and a switch. A steady current through the coil magnetises a steel bar placed inside it, while an alternating current, with the magnet drawn slowly out of the coil, removes unwanted magnetism.

Inside a Long Solenoid

The listing image of the demagnetizing and magnetizing coil shows a long copper winding on a former held between two dark end supports, all fixed to a flat board. A red and a black terminal and an on-off switch sit on the board, and a metal rod is pictured in front of the unit, ready to be slid into the coil.

A current in a long solenoid produces a strong, fairly uniform magnetic field along its axis, proportional to the current and to the number of turns per unit length. A steel bar laid inside has its magnetic domains pulled into line with that field. Steel is a hard magnetic material, so many domains stay aligned when the current is switched off and the bar is left as a permanent magnet. A soft iron bar treated the same way loses most of its magnetism once the current stops, which gives a direct comparison between the two materials. The clock rule tells students which end becomes north: looking at an end of the coil, an anticlockwise current makes it a north pole.

Demagnetising works the other way round. With an alternating current in the coil, the field reverses many times a second; drawing the magnet slowly out along the axis and well away from the end lets the reversing field it experiences fade gradually to nothing, leaving the domains in random directions. Magnetising calls for direct current and demagnetising for alternating current, so the coil’s rated supply, and whether it accepts AC, should be confirmed before use.

Applications

• Magnetising steel bars and needles, then checking their poles with a compass
• Comparing how steel and soft iron keep or lose magnetism
• Demagnetising weakened or reversed bar magnets and magnetised tools
• Applying the clock rule and the right-hand grip rule to predict poles

Care & Handling

• Connect only the supply type and voltage specified for the coil.
• Switch the current on only while magnetising or demagnetising, since a long run heats the winding.
• Check leads and terminals before use and keep the board dry.
• Keep watches, phones and bank cards away from the coil while current flows.

Specifications

Why Choose LabEquip

School physics labs use this coil to make and reset magnets for magnetism practicals, and it lets teachers restore bar magnets that have weakened over years of use. LabEquip supplies it through the STEM kits range alongside the Magnetic Science Experiment Kit, and our contact page can confirm the supply the coil needs.

Frequently Asked Questions

How does the coil magnetise a steel bar?

A current in the solenoid creates a strong field along its axis. Steel placed inside has its magnetic domains pulled into line with that field, and because steel is a hard magnetic material, many stay aligned after the current stops, leaving a permanent magnet.

How is the coil used to demagnetise a magnet?

An alternating current is passed through the coil and the magnet is drawn slowly out along the axis and well away from the end. The field keeps reversing while its strength at the magnet falls gradually to zero, leaving the domains in random directions.

Why does switching off a steady current not demagnetise the bar?

Removing a steady field leaves the aligned domains where they are, which is exactly how the bar was magnetised in the first place. Only a field that keeps reversing while it weakens can scramble the domains.

Which end of the bar becomes the north pole?

Look at one end of the coil: if conventional current flows anticlockwise around it, that end becomes a north pole, and clockwise gives a south pole. The right-hand grip rule gives the same answer, and a compass confirms it.

Why does soft iron lose its magnetism when the current stops?

The domains in soft iron turn easily, so they follow the field while it is on and relax back to random directions when it is removed. That makes soft iron suitable for electromagnet cores and steel suitable for permanent magnets.

What supply does the coil need?

Magnetising needs a steady current and demagnetising an alternating one, so the coil’s rated voltage and whether it accepts AC should be confirmed at enquiry before it is connected.

Last Updated: September 2026