3D Magnetic Field Visualisation

Ferrofluid following magnetic field lines. Image attribution: Gregory F. Maxwell, CC BY-SA 3.0 via Wikimedia Commons

You probably performed the experiment in primary school where you place a sheet of paper over a bar magnet and sprinkle iron filings, give the sheet a tap, and the particles of iron arrange themselves along the magnetic field lines of the magnet. However the iron filings experiment only visualises a slice through the magnetic field. In fact, magnetic fields are three dimensional and the top photo is a 3D version of the experiment using ferrofluid, a colloidal liquid (like milk or emulsion paint) with a suspension of nanoscale ferromagnetic (influenced by magnetism e.g. iron) particles. The ferrofluid is placed on a sheet of glass above a neodymium magnet.

Iron filings on paper following magnetic field lines of a bar magnet. Image attribution: Newton Henry Black, public domain via Wikimedia Commons