CORDELIA MOLLOY / SCIENCE PHOTO LIBRARY CORDELIA MOLLOY / SCIENCE PHOTO LIBRARY
Magnetic field. Iron filings aligned along the magnetic field lines surrounding a bar magnet. The poles are marked north (N) and south (S). The magnetic field induces magnetism in the needle-shaped filings, giving each a north and south pole. The magnetised filings align in thin arcing lines due to the interactions between them, even though the magnetic field itself is continuous. Magnetism is a property of ferromagnetic metals (such as iron, nickel and cobalt) caused by the alignment of the spins of the electrons in the metal.
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