Iron filings show the field of a toroid which is wound through a sheet of Plexiglas.
The toroid is placed on an overhead projector. Iron filings are then sprinkled evenly onto the acrylic stage of the toroid demo. A high current pulse is sent through the toroid, causing the filings inside the toroid's windings to align with the magnetic field.
Toroid, pulse power supply, and overhead projector.