Materials, particularly ferromagnets such as iron, will have their dipoles line up along the direction of an applied magnetic field. The magnetic dipoles of the material's atoms thus create a magnetic field of their own which adds to the net magnetic field. The magnetic field inside a solenoid can therefore be greatly strengthened by using iron for the core rather than air.
where m is called the magnetic susceptiblity, which can be larger than m0. We demonstrate the increased magnetic field by inserting an iron core into a solenoid and showing that it then picks up nails more readily.
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