fig1 shows an excitation coil 4 and a signal coil 7 as electrical components arranged on a plane , according to the prior art . the signal coil 7 is surrounded , for example , by the excitation coil 4 . with regard to the further construction of the example of the excitation coil 4 , the signal core 7 and an evaluation system using a probe , reference should be made to de 197 48 556 a1 , which is expressly intended to be a part of this disclosure . the excitation coil 4 and the signal coil 7 are electrically isolated from one another . the signal coil 7 in this example is in the form of a difference probe . the spatial resolution is governed by the distance between the centres of gravity of the two coil elements , the so - called baseline . the excitation winding 4 surrounds the coil elements of the signal coil 7 symmetrically , for example , so that this ensures compensation for the excitation field . an xxl probe has a baseline of 3 . 3 mm , an excitation coil with 21 turns , and a signal coil with 8 turns . an s probe has a baseline of 2 . 3 mm , an excitation coil with 9 turns , and a signal coil with 5 turns . a probe which , inter alia , comprises an excitation coil 4 and a signal coil 7 is moved in a scanning direction 13 , identified by an arrow , over a surface of a test body 10 ( indicated by a dashed circumferential line ), with the probe 1 coming to rest with a contact surface 37 ( fig2 ) on the test body 10 . by way of example , the test body 10 contains defects in the form of cracks , which influence a magnetic signal in the excitation coil 4 , by which means it is possible to detect the defects in the interior of the test body 10 , and on its surface . fig2 shows a first exemplary embodiment of a probe 1 for electrical measurement methods according to the present invention . a sheet which is flexible , for example , is used as the substrate 16 , which rests directly on the test body . a polyimide sheet is preferably used . the excitation coil 4 and the signal coil 7 are arranged , for example in a planar form , on the substrate 16 , that is to say the coil comprises only one conductor track , which runs only on a plane . the coils 4 , 7 , as electrical components , can be fitted to the sheet 16 by means of a galvanic process or a wet - chemical method . an adhesive 19 , which connects a rear key 22 to the substrate 16 , is , but need not necessarily be , applied to the substrate 16 and to and around the coils 4 , 7 . the rear key 22 is likewise flexible . a ferrite material ( for ferromagnetic signal amplification ) with a permeability μ of up to 100 is preferably used as the material for the rear key 22 . by way of example , at least one electrical supply line 31 for the coils 4 , 7 for a measurement system according to de 197 48 556 a1 is passed through the rear key 22 . a polymer sheet 25 filled with ferrite particles may be used as the rear key 22 . it is likewise possible to use a thin flexible ferrite metal sheet for signal amplification . the polyimide sheet 16 has , for example , a thickness of 25 μm , the copper coil has a thickness of 17 μm , the adhesive extends over a thickness of about 30 μm , and the polymer sheet that is filled with ferrite extends over a thickness of 200 to 600 μm . this layer stack remains sufficiently flexible to allow the layer stack to be matched to different radii of curvature of the test body 10 , for example , 50 mm and more , without any problems . fig3 shows a further exemplary embodiment of a planar probe 1 designed according to the invention . the rear key 22 can also be ensured by means of an encapsulation material 34 , in which ferrite powder is mixed . the mean diameter of the ferrite particles is , for example , about 10 μm . the encapsulation compound is and remains easily plastically deformable after a curing process , thus ensuring that the probe 1 is permanently flexible . an encapsulation probe such as this can also be used in order to produce a rigid probe 1 for specific curved surfaces . in this case , an encapsulation compound 34 is used which can be cured in a state in which it is deformed in this way , such that it can subsequently be plastically deformed only with difficulty , and is thus permanently matched to the contour of specific test bodies 10 . the advantage of the method in this case is that a flexible probe 1 is first of all matched to a surface of a test body 10 without any major effort , and the encapsulation compound 34 is subsequently cured , so that there can be no air gap between the contact surface 37 of the sheet 16 and the curved surface of the test body 10 to corrupt the measurement result . as an electrical measurement method , the probe 1 which , by way of example , has two coils 4 , 7 or only one coil as well as the ferromagnetic signal amplification 22 , can be used for eddy current measurement which serves , for example , to detect defects in or on metallic components 10 .