Magnetic field line
The actual magnetic field itself does not have "lines" in it; the "lines" are solely the iron filings becoming polarized themselves, reacting to each other and the field, lining up N and S with respect to each other in the field. If you could see the actual fields of force, it would be shaded and gradient, with a heavier, thicker shade near the stronger part of the magnet, fading the further you get from the source. And in all 3 dimensions, which iron filing demonstrations cannot reproduce. Ferrofluids will react in all 3 dimensions and can more accurately reproduce a field, except for gravity creating a weight restriction. Holding a strong magnet in front of a CRT-type monitor with a white screen can also give a representation of the fields, without any "lines of force" visible. The issue with using ferro/magnetic materials to view a field is, the materials themselves become magnetized and change the original field to include their own influence.