Abstract: The height position of three-phase electrodes R, S, T immersed in a resistive medium is controlled first in dependence of the values for the equivalent star network load impedances Z.sub.RO, Z.sub.SO, Z.sub.TO of the electrodes, these impedances being calculated on the basis of measured values representative of valid delta network load impedances Z.sub.RS, Z.sub.ST and Z.sub.TR of the electrodes. The height positions of the electrodes are then equalized by selective adjustment of the electrical conductivity of the resistive medium at respective electrodes, preferably by using the so-called derivative method.