This overhead projector demonstration uses the iron filings. The first part of this demonstrates that the magnetic field inside a solenoid is parallel to the z-axis of the solenoid and is proportional in strength to the number of coils per unit distance.
The second part investigates the effects of paramagnetic and ferromagnetic materials on the magnetic fields.
If a paramagnetic material (copper shown below) is added, there is no little or no change in the field lines within the solenoid.
If a ferromagnetic material (soft iron is shown below) is added, it has a strong influence on the magnetic field lines.