In this experiment, students investigate the magnetic flux density B of a multi-layer coil along the central axis z. Based on their measurements, they should recognise that the magnetic field is not homogeneous compared to the single-layer air coils used in theory and in most experiments in physics lessons. The coils used here, with 500 and 1000 turns, are very well suited for demonstrating these inhomogeneities. The measurement results can also be used to determine the vacuum permeability µ0 very accurately using a simplified fitting function. The parameters N, L, ra and ri are fixed in the fits.