3 Dimensional Magnetic Field Demonstrator 3-D Magnetic Filed Demonstrator uses iron filings in a clear acrylic case to show the full magnetic field of the included test magnet. Simply shake the demonstrator up and down and the iron filings will form around the magnet in stunning detail. Filings are completely sealed in the case case at all times, whether closed or open, and the filings never need to be replaced. The case hinges opens to reveal the cylinder magnet inside the unit. Opening the case also allows the viewer to simultaneously see the lines of force in 3-D and a 2-D cross section.