referring now to the figures of the drawing in detail and first , particularly , to fig1 thereof , there is shown a measuring head 1 for measuring an installed part , in particular a fuel rod of a fuel assembly channel or a spacer of a fuel assembly , in a nuclear engineering installation . the measuring head 1 is designed in such a manner that it is possible both to determine a contour or geometry characteristic value of an object to be examined , i . e ., of the workpiece , and to determine the thickness of an oxide layer on the surface of the object which is to be examined simultaneously and in one measuring operation , in the manner of a combined measurement . to determine a contour or geometry characteristic value , the measuring head 1 comprises a sensor housing 2 on which a sensing tip 4 is disposed . the sensing tip 4 is made from diamond , but may also be in multi - component form using a material with a suitably selected hardness . to determine a contour or geometry characteristic value of an object 7 , which may in particular be a fuel rod or a spacer in a fuel assembly , the sensing tip 4 can be guided along the object , in mechanical contact therewith . any change in the contour or geometry at the surface of the object to be examined results in a change in the position of the sensing tip 4 and therefore the measuring head 1 overall , in the direction x indicated by the double arrow 8 . to provide a characteristic measured value for a change in position of this nature , the measuring head 1 is arranged at the free end 10 of a sensing arm 12 . for its part , the sensing arm 12 is suitably secured to a carrier device at its other end , which is not illustrated in more detail in fig1 . in the exemplary embodiment shown in fig1 , the sensing arm 12 is made from spring steel sheet . in this case , a deflection of the free end 10 of the sensing arm 12 as a result of a change in position of the measuring head 1 in the measuring direction x leads to bending or deformation of the spring steel sheet . this can be recorded quantitatively by way of a strain gauge 14 mounted on the surface of the sensing arm 12 . therefore , an assembly of this type makes it possible to record even relatively minor changes in the position of the measuring head 1 in the direction x with a high resolution . as an alternative , or in addition , the sensing arm 12 may also be provided with a bending joint , the bending angle of which can be recorded using a suitable bending angle sensor . in that case , the position of the measuring head 1 in direction x can be determined by combined analysis of measured values of the strain gauge 14 and the bending angle sensor . the measuring head 1 which is in this way suitable for providing a contour or geometry characteristic value is furthermore also configured to measure the thickness of an oxide layer 20 on the surface of an object 7 to be examined , carried by a metallic base material 22 of the object 7 . for this purpose , a layer thickness measuring probe 30 is integrated in the sensor housing 2 of the measuring head 1 . the layer thickness measuring probe 30 comprises a coil arrangement 32 , which is connected , via signal lines 34 guided through the sensor housing 2 and through the sensing arm 12 , to an eddy current detector 15 . the coil arrangement 32 is cast into a carrier sleeve 38 by way of a potting compound 36 , and the sleeve 38 is in turn secured in the sensor housing 2 . the coil arrangement 32 is positioned in such a manner that its symmetry or center line is congruent with that of the sensing tip 4 . therefore , the layer thickness measuring probe 30 is disposed inside the sensor housing 2 behind the sensing tip 4 . the layer thickness measuring probe 30 is configured to determine the thickness of the oxide layer 20 on the basis of what is known as “ lift - off effect .” for this purpose , the coil arrangement 32 for supplying an eddy current detector is connected on the input side to a high - frequency generator via a wheatstone bridge circuit . the output - side part of the bridge circuit is connected to an eddy - current measuring assembly by way of a transmitter . to determine the layer thickness of the oxide layer 20 , the coil arrangement 32 is moved into a defined position close to the oxide layer 20 . the defined position of the coil arrangement 32 with respect to the oxide layer 20 is in this case ensured by virtue of the fact that the coil arrangement 32 , on account of its fixed installation in the sensor housing 2 , adopts a defined and constant distance from the front edge of the sensing tip 4 . the sensing tip 4 in turn — not least in order to carry out the contour measurement — is brought into direct physical contact with the surface of the oxide layer 20 , and consequently exact positioning of the coil arrangement 32 with respect to the surface of the oxide layer 20 is automatically ensured . when measuring the layer thickness of the oxide layer 20 , the coil arrangement 32 is fed with a high - frequency input signal . the magnetic field generated by the coil arrangement 32 operated in this way produces an eddy current in the metallic base material 22 of the object 7 that bears the oxide layer 20 . the resulting eddy current in turn affects the impedance of the coil arrangement 32 . the level of this effect is dependent on the distance of the metallic carrier layer 22 from the coil arrangement 32 . for its part , this distance is given by the sum of the distance of the coil arrangement 32 from the front edge of the sensing tip 4 and the thickness of the oxide layer 20 . the bridge circuit used to operate the coil arrangement 32 therefore supplies as its output signal a voltage signal which for its part is characteristic of the thickness of the oxide layer 20 and can therefore be evaluated to provide the latter . the measuring head 1 is therefore suitable for the simultaneous determination of the contour , on the one hand , and the thickness of a surface oxide layer 20 of the object 7 to be examined , on the other hand . therefore , the measuring head 1 can particularly advantageously be used for time - saving measurement of installed parts , in particular of fuel rods , fuel assembly channels , spacers or other structural parts in a nuclear engineering installation in order to allow reliable estimation of their ageing state or expected remaining service life . for this purpose , the measuring head 1 , as shown diagrammatically and in side view in fig2 and in plan view in fig3 , can be used in a measuring device 50 which for its part in turn forms part of a fuel rod testing assembly 52 . the measuring device 50 shown in fig2 and 3 comprises two sensing arms 12 , at the free , laterally deflectable end 10 of which in each case one of the measuring heads 1 is arranged . in this case , as can be seen in particular from the plan view shown in fig3 , the sensing arms 12 are arranged opposite one another in the style of a fork and are therefore suitable for engaging around a fuel rod 54 which is guided between them and is to be tested . the measuring heads 1 are in this case arranged opposite one another , so that the fuel rod 54 can be scanned on both sides . on account of this arrangement of the measuring heads 1 opposite one another , it is possible to determine a characteristic value for the diameter of the fuel rod 54 . the sensing arms 12 are arranged on a common carrier body 56 , which for its part is mounted on a number of guide rolls 58 which can be guided along the fuel rod 54 . the guide rolls 58 in each case comprise a central roll body 60 which is delimited on both sides by in each case one guide disk 62 . the guide disks 62 have a greater diameter than the diameter of the roll body 60 . a roll body 60 which rolls along the surface of the fuel rod 54 is therefore automatically centered with respect to the fuel rod 54 which is to be examined by the guide disks 62 which delimit it . the fuel rod testing assembly 52 together with its measuring device 50 can be moved along the fuel rod 54 , in the longitudinal y - direction of the fuel rod 54 indicated by the double arrow 64 , by means of a drive device . as a result , if necessary the fuel rod 54 can be scanned along its longitudinal direction y . this scanning on the one hand , in the style of a profile measurement , provides contour measured values on the basis of the sensing tips 4 of the measuring heads 1 , which in turn can be converted into position - dependent diameter characteristic values for the fuel rod 54 . this scanning in this case takes place along a scanning track indicated by the line 66 . furthermore , during the scanning , the layer thickness measuring probes 30 of the measuring heads 1 provide position - dependent characteristic values for the thickness of an oxide layer which may surround the fuel rod 54 . the corresponding measured values are in this case likewise recorded along the sensing track 66 directly via the corresponding sensing tip 4 on the basis of the positioning of the corresponding coil arrangement 32 . therefore , scanning of the fuel rod 54 which takes place simultaneously for both relevant parameters in this way results in a particularly extensive set of data for the fuel rod 54 being provided within a relatively short measuring time , on the one hand , while on the other hand it is also possible to analyze any correlations between the local layer thickness of the oxide layer and the local diameter value of the fuel rod 54 on the basis of the local correspondence between the respective measurement parameters . alternatively , the measuring head 1 may also be used in a measuring device 70 for a spacer 72 of a fuel assembly , as diagrammatically indicated in plan view in fig4 . the measuring device 70 shown in fig4 in this case likewise comprises two sensing arms 74 , at the free , laterally deflectable end 76 of which in each case one of the measuring heads 1 is disposed . in this measuring device 70 too , the sensing arms 74 are arranged opposite one another in the style of a fork and are therefore suitable for engaging around the spacer 72 which is guided between them and is to be tested . the sensing arms 74 of the measuring device 70 are arranged on a guiding and advancing unit 80 as a common carrier body with the components thereof which are customarily provided .