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Region around a magnet, electric current, or changing electric field in which magnetic forces are observable. The field around a permanent magnet or wire carrying a steady direct current is stationary, while that around an alternating current or changing direct current is continuously changing. Magnetic fields are commonly represented by continuous lines of force, or magnetic flux, that emerge from north-seeking magnetic poles and enter south-seeking poles. The density of the lines indicates the magnitude of the field, the lines being crowded together where the magnetic field is strong. The SI unit for magnetic flux is the weber.
This entry comes from Encyclopædia Britannica Concise. For the full entry on magnetic field, visit Britannica.com.