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 will therefore create a magnetic field of their own which will strengthen the net magnetic field. The magnetic field inside a solenoid will be strengthened if iron is used for the core rather than a vacuum.
where m is the magnetic susceptiblity which can be larger than m0. We will demonstrate the increased magnetic field of a solenoid by replacing the core with iron, and show that is picks up nails more readily.