Invented in 1824 by the British physicist William Sturgeon, electromagnets are a fixture of modern life, appearing in loudspeakers, motors, magnetic resonance imaging (MRI) machines, maglev trains, and particle accelerators.
When an electric current flows through a wire, it creates a magnetic field around the wire. So when the wire is coiled, the magnetic field becomes concentrated in the coil’s hole. This is an electromagnet: a device in which an electric current produces the magnetic field. When the wire is coiled around a magnetic material, the material — called the core — amplifies the strength of the magnetic field.
This is because ferromagnetic metals like iron already contain small magnetic fields inside their bulk produced by its atoms. Because these fields point in random directions, the fields cancel themselves out. When the iron is used as the core in an electromagnet and a current is passed, the magnetic field it produces penetrates the iron’s bulk and forces the tiny fields to align along the external field’s direction. This way, the domains’ fields add to the external field, producing a stronger field overall.
A magnetic field will be produced as long as a current flows through the coil. When the current is switched off, some core materials remain weakly magnetised. In a superconducting electromagnet, such as in an MRI, superconducting wire is coiled around a core to produce magnetic fields of up to 30 tesla (600,000-times as powerful as the earth’s). In Bitter electromagnets, current flows in wires coiled through a helical stack of electromagnets, producing magnetic fields of up to 40 tesla.
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