The equivalent capacitance of the arrangement is given by the overlapping areas which here are the same, i.e. the part of the first and second electrodes 4 1, 4 2 overlapping first and second sections 3 1, 3 2 of the second electrode means and, since there is no misalignment, the symmetrical and identical overlapping areas of the intermediate section 33 and the first and second electrodes 4 1, 4 2