by being snapped in directly adjacent to the appendage 5 a rib 7 in the form of a partial spherical layer is disposed on every arm 3 the external diameter of the ribs 7 corresponds to the internal diameter of the spherical shell the arms 3 the appendage 5 and the ribs 7 are made of one piece the arms 3 are shaped convexly at their lateral flanks in an area close to the center an intermediate shelf 1 b is disposed as a separate part of the housing 1 underneath the arms 3 between the upper and the lower side of the housing 1 and parallel relative to these sides an upwardly pointing tip 8 in the shape of cone with a cylindrical base is formed centrally on the intermediate shelf 1 b the opening angle of the tip 8 is smaller than that of the depression 4 in this context the difference between the opening angles is determined dependent on the switching distance required a cross shaped leaf spring 9 with four tongues is disposed between the intermediate shelf 1 b and the arms 3 in such a way that each of the tongues extends parallel to an associated arm 3 and exerts a force acting upon it in the upward direction here the leaf spring 9 is clamped onto the cylindrical base of the tip 8 four upwardly pointing pins 10 are formed on the intermediate shelf 1 b the pins 10 are disposed such that there is one of the pins 10 in each corner formed by the arms 3 the pin contacting the two adjacent arms.