In the international tokamak reactor (INTOR), the problem of the passive control of the vertical instability is to be solved by means of suitably shaped saddle coils to be embedded in the blanket structure. The efficiency of such a system depends on the characteristics of the passive conductors and on the plasma equilibrium as well as on the type of plasma displacement assumed. To cover the physical uncertainties caused by the model assumptions for the plasma with respect to the motion on a slow time scale (of the order of several tens of milliseconds) corresponding to efficient passive stabilization, four different plasma displacement models are considered and compared with each other. A stability analysis is performed using the energy principle, expressed in circuital form. The results of the INTOR analysis are presented and discussed, showing in particular that under very general conditions the optimum stabilization efficiency is obtained for passive conductors situated at ∼60 deg above and below the horizontal midplane at the outboard side. The effect of the geometric parameters of the saddle coils (e.g., area and shape of the cross section, toroidal segmentation, etc.) on the stabilization efficiency is investigated; a parametric study of these dependences is presented. General conclusions applicable to INTOR are drawn.