Gradient of the stellar magnetic field in measurements of hydrogen line cores

We report the observed systematic differences in longitudinal magnetic field values, obtained from measurements of metal lines and the core of the H_β line for a number of Ap stars, having strong global magnetic fields. In overwhelming majority of cases the magnetic field values, obtained from measurements of hydrogen lines cores, is smaller then the ones obtained from metal lines. We discuss some possible explanations of this effect, the most probable of which is the existence of the gradient of the magnetic field in stellar atmospheres.