Although we cannot see magnetic field lines, we can detect them using iron filings. The tiny pieces of iron line up in a magnetic field.

The iron filings in the image become induced magnets when they are near the bar magnet. They lose most or all of their magnetism when they are removed from the magnetic field.

Here, we describe how to use readily available materials ... by a similar three-dimensional field. This can be "seen" in two dimensions if iron filings are sprinkled on a sheet of paper placed ...

Iron filings ... flow of iron filings. [BitBlt_Korry] calls his creation “「時場(じば)」”. Google translates this to “Jiba”, which means “magnetic field”.