the single piece steel ring 16 there can be also provided a steel ring which consists of a plurality of segments 33 which are arranged at spacings relative to each other in order to illustrate the foregoing in fig2 the steel ring 16 is shown separated along the coordinate axis x in order to afford a uniform spacing 26 of the segments 33 with respect to each other spacers 34 are arranged intermediate the individual segments 33 in the arrangement illustrated in fig1 the connecting member 4 for the probe elements 21 by means of the bearing locations 26 and 27 are maintained in a centralized contact against the mounting member 2 this contacting is occasioned by the permanent magnet 14 which through the end surface 18 of the steel ring 16 draws the connecting member by means of electromagnetic force in the direction of the coordinates z against the mounting member 2 for effectuating the release of the mounting member 4 the coil 10 is supplied with electric current through the electrical cable 28 until an opposite magnetic field is generated in comparison with that from the permanent magnet 14 this oppositely directed magnetic field must reach at least the magnitude of that of the magnetic field from the permanent magnet 14 as a result there is.