Horseshoe Magnet with Keeper: magnetic field lines connect North to South poles from one side of the U shaped magnet 🧲, through the air, to the other- and magnetic viewing film reveals the strong fields of this vintage magnet. However, when a bar of soft iron is attached, the magnetic field lines prefer to pass through the bar, and the field is then concentrated in the bar and significantly reduced in the surrounding air- basically shutting off the field around the magnet as seen by the viewing