This can be used to show that sensitive apparatus can be shielded from magnetic fields, or to reinforce the idea of field lines following a path and diverting through a piece of iron.
Apparatus and materials
Nail or paper clip
Iron plate or rod
Lab jack and mass
Health & Safety
Hold a powerful magnet (e.g. Eclipse Major) in a clamp above the desk. Attach a small magnetic object such as a paper clip or small nail to a heavy mass with a piece of thread. Place the mass on the lab jack under the magnet and lift the nail. The thread should be long enough that the magnet pulls the nail, but the nail is not allowed to touch the magnet. Use the lab jack to adjust the height so the nail is on the verge of falling.
Now pass another piece of magnetic material, such as a sheet of iron or a rod of iron or mu-metal, between the nail and the magnet. The nail should fall as the rod passes between it and the magnet. Demonstrate that a non-magnetic material, such as aluminium, plastic or glass has no effect.
At a low level this activity can simply serve to emphasize the difference between magnetic and non-magnetic materials, and the idea that forces can act at a distance, even through other objects (such as plastic).
At a higher level students may expect the iron to strengthen the field. It does - internally. And it also strengthens the field at its tips. Carefully drawn field lines such as those in and around a classic horseshoe magnet can be used to show the field lines being drawn away from the nail, and so the field gets weaker there, not stronger.
This has important consequences for shielding from electromagnetic radiation - such as from mobile phones, or stopping computer data being picked up remotely. However, electromagnetic radiation can be shielded just by a conductor, e.g. aluminium.
This experiment was submitted by Ken Zetie, Head of Physics at St Paul's School in West London. He is on the editorial board of Physics Education and regularly contributes to Physics Review.