Magnetic susceptibility
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.