The introduction of a magnetic probe into a plasma causes a perturbation of the current density distribution and with that a perturbation of the magnetic field measured by the probe. There are two different causes of this perturbation. On the one hand the changes in the particle temperature and particle density influence the conductivity (conductivity error). On the other hand the surface of the magnetic probe introduces a variation of the boundary conditions and therefore requires a change in the current density field (boundary error). Assuming stationary conditions our calculation considers the „boundary error”︁. A general relation is calculated and evaluated with the help of a digital computor for a number of characteristic profiles. In all cases we find strong deviations in the center and near the wall of the discharge. In particular our results contradict the common assumption that strong deviations should only be expected in the presence of magnetic field inhomogenities. An iterative procedure is developed which enables us to correct measured field distributions with respect to the „boundary error”︁.