in fig1 a measuring device according to u . s . pat . no . 3 , 976 , 934 is partially shown in a cross section . a conductor 30 to be tested contains a normally conducting core 31 which is surrounded by a superconducting layer 32 . an aluminum wire of circular cross section on the outside of which a niobium layer is applied may be provided as the conductor core . however , the superconductive material need not complete enclose a normally conducting core with a thin outer layer . it may also be distributed over part of the conductor cross section or over the entire conductor cross section . the conductor 30 is assumed to be in the superconducting state and is moved through an external magnetic field . this magnetic field is illustrated by individual field vectors 34 , which are disposed substantially perpendicular to a plane in which the axis of the conductor lies . because of the generally relatively large three - dimensional extent of the superconducting layer 32 of the conductor 30 in the direction of the field vectors 34 , it is advisable to provide two hall probes 36 and 37 . these two hall probes are arranged in the direction of the field diametrically opposed with respect to the conductor near the surface of the conductor . the hall effect voltages generated in them are advantageously added . the following simplifying assumptions can be made to estimate the effect to defect areas on the measuring signal produced . as shown in the figure , a defective part of the conductor layer 32 of the conductor 30 of size δα is assumed to be at an angle α to the direction of the field vectors 34 and carry no current . the change of the hall probe signal associated therewith is shown on the diagram of fig2 . the calculated curves of this diagram are based on an aluminum wire of 2 mm diameter as well as on different distances z of the hall probe from the surface of the wire . the value b z designates the sum of the fields generated by the induced current in the undisturbed conductor at the location of the two hall probes ; and b z ( α ) designates the sum of the missing fields of the conductor area δα at the angle α also at the location of the two hall probes . in the diagram of the figure , that fraction of the field generated by the induced current as well as the superconducting layer are reduced by the same fraction . values larger or smaller than 1 indicate more or less relative change of the field generated by the induced current in comparison to the change of the superconducting layer . as can be seen from the diagram of the figure , a defect at an angle α of about 90 ° can be registered very well , while a corresponding defect at an angle α = 0 or 180 ° causes practically no signal change . for this reason , and as shown schematically in fig3 the magnetic field according to the present invention , through which the conductor 30 must run , comprises two magnetic field regions 39 and 40 which are staggered in the longitudinal direction of the conductor . the field vectors , 41 of the magnetic field region 39 are angularly displaced relative to the corresponding field vectors 42 of the magnetic field region 40 by about 90 ° in the circumferential direction of the conductor . thus , the defects occurring in a superconducting layer 32 , i . e ., particularly cracks , for instance , several centimeters long , in the axial direction can lead distinctly to signal changes at the hall probes , although the zone designated with δα according to fig1 may be very small . when the conductor 30 is moved through the magnetic field regions 39 and 40 , shielding currents which are indicated by lines 43 and 44 , are induced in its superconducting layer 32 . each magnetic field connected with these shielding currents and indicated by individual field vectors 46 and 47 , respectively , is thus superimposed on the magnetic fields 41 and 42 , respectively . the magnetic field changes connected therewith can be measured according to fig1 by two hall probes for each field of which in the figure the hall probes associated with the magnetic field region 39 are designated 49 and 50 and the hall probes associated with the magnetic field region 40 are designated 51 and 52 . in fig4 a longitudinal cross section through part of a measuring apparatus according to the present invention is shown . the conductor 30 to be tested , which is cooled down to superconductivity , is introduced into a cryostat ( not shown in the figure ) which is filled , for instance , with liquid helium . it is brought out of the cryostat again in the opposite direction by means of a deflecting device 54 , e . g ., a track bent in semicircular shape . on the way along the deflection track 54 , the conductor runs through two magnetic field regions 39 and 40 according to fig3 . the magnetic fields of these magnetic field regions are generated , according to fig4 for instance , by two magnetic field coils per field region which are arranged on both sides of the conductor and which may be normally conducting or superconducting eletromagnets . the two magnet coils 56 and 57 assigned to the magnetic field region 39 are shown in the figure in a longitudinal cross section . of the magnet coils assigned to the magnetic field region 40 , only one magnet coil 58 is visible in cross section in the figure . this coil just covers up the magnet coil opposite it . the two magnet coils of this magnetic field region correspond as to their geometric dimensions , for instance , to the two magnet coils 56 and 57 . the magnetic field produced by the magnet coils 56 and 57 is aligned approximately perpendicular to the conductor axis and approximately radially with respect to the semicircular deflection device 54 . the two magnet coils of the magnetic field region 40 are angularly displaced relative to the magnet coils 56 and 57 by about 90 ° in the circumferential direction of the conductor axis and approximately parallel to the axis 60 , of the semicircular deflection device 54 . between each of the pairs of magnet coils of the two magnetic field regions 39 and 40 , two hall probes are arranged in the direction of the generated magnetic fields on both sides of the conductor 30 . in the view of fig4 the two hall probes 49 and 50 associated with the magnet coils 56 and 57 can be seen , while the hall probe 51 associated with the magnet coil 58 just covers up the opposite hall probe on the other side of the conductor . for a diameter of 2 mm of the conductor 30 , about 1 mm is advantageously chosen as the distance of these hall probes from the conductor surface , while a distance of 1 . 5 mm is advantageously provided for a 3 mm conductor . the hall voltages of the two hall probes of each pair of magnet coils can advantageously be added and compensated about the hall voltage of the applied field . these hall voltages u h of the two magnetic field regions 39 and 40 , cancelled in this manner , can now be fed to a multiple recorder . thus , the measuring signals from the two magnetic field regions can be recorded independently of one another . as an example of such recording , a corresponding graphic recording is shown in fig5 for several meters of a conductor 30 with a diameter of about 3 mm . an aluminum wire with an outer niobium layer was used as the conductor . this niobium layer of the conductor section was damaged by filing at some defined points in such a manner that the underlying aluminum core became visible . thereupon , this damaged conductor in wire form was pulled through two magnetic field regions 39 and 40 in an apparatus according to fig4 . in the graphic recording shown in the figure , the hall voltage u h def produced at the hall probes is reproduced as a function of the location x along the conductor . the percent figures on the ordinate serve as a scale and are referred to the hall voltage u h of the undamage conductor . the lower curve shows the signal of the first magnetic field region 39 . an external magnetic flux density of 0 . 24 tesla was chosen for the two corresponding pairs of magnet coils . the distance between the two pairs of magnet coils in the lengthwise direction of the conductor was about 117 mm . in the left - hand part of the graphic recording according to fig5 the signals designated d which were produced by the intentionally generated defects are seen . in the center of the upper curve , which is associated with the second magnetic field region 40 , spike - shaped signals are clearly recorded , which are not present in the lower curve or only vestigially so . these are known as flux jumps and are brought about by magnetization reversal processes in running through the two magnetic field regions . it has now been found that these signals disappear if the magnetic field of the first magnetic field region 39 is reduced slightly . however , if the field is increased , corresponding signals appear in a denser sequence one after the other . it may therefore be advisable to choose different magnetic field strengths for the two pairs of magnet coils flux jumps are then avoided to a large extent . in addition , the advantage is obtained that the first measuring device , which is associated with the magnetic field region 39 , is relatively insensitive and registers mainly grave defects , while the measuring probes associated with the second field region 40 can react with high sensitivity even to very small defects . in the embodiment according the figures above , the assumption of a solid conductor of circular cross section and with a superconducting surface was made . however , the conductor may also be a tubular , e . g ., a hollow conductor , or may have another geometric shape , for instance , an elliptical cross section . furthermore , more than two magnetic field regions e . g ., 3 field regions , may be provided , the field directions of which are rotated relative to each other by about 60 ° in the circumferential direction of the conductor about the conductor axis .