Abstract: An improved eddy current probe system and method for simultaneously detecting different types of flaws at different depths within a metallic wall, such as a section of Inconel tubing, is disclosed herein. The system comprises a current generator for generating alternating currents of substantially different frequencies, a probe head including first, second and third concentrically arranged coils in separate communication with the current generator, shielding material disposed between the coils for preventing cross talk between each coil and the pulsating magnetic field of the coils adjacent to it, and a detector circuit which may include an inductive bridge for providing an electrical output representative of the impedance changes in the respective coils. In operation, each of the coils conduct currents having substantially different frequencies, the highest frequency being conducted by the smallest-diametered coil and the lowest frequency being conducted by the largest-diametered coil.

Abstract: An eddy current probe capable of detecting localized defects in a ferromagnetic tube is disclosed. The probe employs a transmit coil assembly and a receive coil assembly. Means for magnetically saturating the tube near the transmit and receive coil assemblies is also disclosed.

Abstract: An electrical defect testing device for a non-magnetic test object, for example crack detection on hot continuously cast billets, is provided with means for detecting and suppressing the influence of disturbing magnetic regions of the object (e.g. regions of cold oxide scale). The device comprises at least one sensing transducer for harmful faults which is adapted to move relative to the test object. At least one detector is associated with the transducer for identifying the presence of magnetic material, and a blocking circuit is provided which, directly or indirectly, is controlled by the magnetic detector and is used for suppressing signals originating from a fault vector, for example a signal indicating a harmful fault. The presence of magnetic material can be detected using an eddy current technique.

Abstract: An eddy current testing device for inspecting test objects, for example billets, with respect to surface defects, for example surface cracks, comprises at least one transducer/sensor, for example a surface transducer or surface transducer arrangement, which is made to scan the surface of the test object. The device is characterized in that the effect of harmless surface blemishes on the testing device is suppressed, at least partially, by compensating for the sensitivity characteristic, in the scan direction of the transducer/sensor, with respect to the surface defect.

Abstract: Defect detecting equipment includes a device which detects the presence of, for example, disturbing magnetic material such as cold oxide scales in a test object, in order thus to control, for example, that disturbances of a magnetic origin are not confused with surface cracks without being discovered. By utilizing eddy current techniques, phenomena which are harmless to the process can be separated from dangerous surface cracks, thus avoiding the scrapping of crack-free test objects. The invention is based on signal-processing at least two signals, for example H1 and L1, originating from a test object sensing tansducer, by means of a vector transformation method, and distinguished between harmful and harmless phenomena (e.g. cracks and oxide scales) as a function of a comparison of signals, S1 and S2l , from at least two transformation blocks.

Abstract: A wire rope tester which can detect broken wires, area reductions, or wire contact irregularities. The tester includes two permanent magnetic stacks which produce axial adjacent and opposed DC magnetic fields which magnetize the rope. Broken wires are indicated by flux variations when the flux density is at saturation; area reductions are related to flux variations; also at saturation flux density; and wire contact pattern changes produce detachable eddy current flux variations when the flux density gradient is constant.

Abstract: A signal processing method in an electromagnetic induction test. A test object is subjected to an electromagnetic induction by use of N (N.gtoreq.1) kinds of test frequencies to obtain M (M.ltoreq.2N) kinds of phase detection outputs corresponding to respective states of the test object. The phase detection outputs are then sampled at a plurality of measuring points so that there exists n (n<M) kinds of disturbance factors of which a portion contribute to the electromagnetic induction test. The phase detection outputs have a variance greater than a variance of a target parameter of the test object and contribute to the electromagnetic induction test. A first coordinate axis Z, is determined in a direction in which a variance of a distribution of the group of the measuring points comes to be a maximum in an M-dimensional space formed with the coordinate axes associated with the M kinds of the phase detection outputs. The remaining coordinate axes are likewise determined.

Abstract: Method and apparatus for identifying and classifying steels in which a steel specimen is presented to a coil which receives an excitation current to produce an electrical output dependent on an induction value of the specimen. An excitation current is applied to the coil at each of a plurality of excitation frequencies, and a respective value of the specimen at each of those frequencies is determined. The value of a discriminant function is derived for each of a plurality of possible steel grades, with the function being of the form a.alpha.+b.beta.+c.gamma.. . . where .alpha.,.beta.,.gamma.. . . are attributes of the specimen comprising the induction values and a, b, c . . . are predetermined coefficients reflecting the degree to which the respective attributes are effective to discriminate that possible steel grade from the others.

Abstract: A method and apparatus for use with a measuring and/or control device having at least one sensor based on eddy current induction and movable over a test piece and supplied with at least two carrier frequency signals. A determination is made on one of the carrier frequency signals with respect to those signal changes obtained at the transistion from measuring against the test piece to measuring against essentially empty space. When such signal changes occur an edge signal is generated which indicates that the measuring and/or control device has passed over an edge of the test piece.

Abstract: A method and an apparatus for the detection of slag co-flowing within a sam of molten steel being poured from metallurgical vessel, more particularly during continuous casting, makes it possible to detect even a small quantity of slag in the emerging flow of molten metal without requiring the removal of the shielding for the pouring stream, or without interfering with the pouring, by measuring changes in the electrical conductivity of the pouring system by means of electromagnetic fields. To this end, one or more transmitter coils and receiver coils are mounted fixedly around the pouring stream. The transmitter coils are fed with a current containing several frequencies, and the magnitude and phase position of the voltage induced in the receiver coils are evaluated frequency-selectively so as to increase the sensitivity, the measuring transducer being operated in a bridge circuit.

Abstract: An eddy current probe capable of detecting localized defects with 100% circumferential coverage in tubes, tubes under support plates etc., is disclosed. The probe employs multiple receiver coils with each excitation coil, operating in transmit-receive mode in which alternate receiver coils are electromagnetically polarized in opposite directions. The probe detects localized defects and eliminates concentric variations.

Abstract: An eddy current probe capable of detecting defects independent of orientation, with complete circumferential coverage in tubes, tubes under support plates, etc., is disclosed. The probe employs multiple coils, operating in transmit-receive mode, in which alternate coils are electromagnetically polarized in opposite directions. The probe detects localized defects and eliminates concentric variations.

Abstract: A magnetic field is produced by way of an emitter placed close to the surface of a product to be controlled and a receiver separate from the emitter is placed, with respect to said emitter, on the one hand, in such a way as to be aligned with the emitter in a direction corresponding to the general orientation of the defects to be detected on the surface of the product, and on the other hand, so as to avoid any substantial direct influence from the eddy currents generated by the emitter when the product surface is defect-free, so that a significant signal is only picked up by the receiver in the case of a deviation of said eddy currents in its direction due to a discontinuity in the surface of the product. The invention finds an application in the detection of surface defects in semi-finished metallurgical products and in particular continuously cast steel products.

Abstract: A method and apparatus for determining the extent of axially extending defects in ferromagnetic elements, such as tubing comprising a continuous string for use in an oil or gas well is disclosed. The tubing trip tool detects axial defects, such as sucker rod wear during removal of the tubing from the well. A saturating magnetic field and a fluctuating magnetic field are applied to the tubing and the magnitude of the induced fields and the changes are measured to quantify defects in the tubing.

Abstract: Method and apparatus for performing eddy current flaw inspection on workpiece material such as steel bar stock. A drive mechanism propels a workpiece longitudinally along a workpiece path. An array of electromagnetic transmitter coils is disposed about the workpiece path. An electromagnetic source energizes the transmitter coils under control of multiplexing circuitry. The multiplexing circuitry and source actuate respective transmitter coils in a predetermined iterative sequence. The sequential actuation induces eddy currents in the workpiece at a succession of locations extending iteratively about the workpiece circumference. Circuitry coupled to multiple, differentially wound sensing coils detects variations in eddy current produced in the workpiece and utilizes these detected variations to produce flaw indicating signals.

Abstract: Apparatus for electromagnetically testing pipeline walls of steel or the like ferromagnetic material which is equipped, for example with a magnetizing coil for leakage-flux measurements, can be extended for ultrasonic measurement which is inductively excited and inductively measured with regard to the travel time as far as a reflecting surface and back, in the disposed in one or more of the pole regions there is a high-frequency current coil for the transmission and/or reception of steep-edged waves which are inductively coupled at the wall of the pipeline. The associated method starts from a stationary magnetic field in which high-frequency current pulses are produced in order to initiate vibrations in the pipe wall through induced currents. The travel time of these vibrations to a reflecting point and back can be measured in order to obtain an indication regarding the intact thickness of the pipe wall.

Abstract: A non-contacting, eddy current, measuring method for determining the thickness and temperature of a moving metal sheet as in a metal rolling operation. Two separate magnetic fields are generated by applying two voltages of differing frequencies to a primary winding on one side of the sheet to induce two voltages in an opposed secondary winding on the other side of the sheet. The generated voltages are used to determine the calibration constants required to calculate the thickness and temperature of the moving sheet.

Abstract: A method for determining whether a test sheet, such as a banknote, is a genuine reproduction of a reference sheet having a magnetic characteristic such as being printed with a ferromagnetic ink. The method comprises sensing for a magnetic effect, such as a magnetic field, with a suitable detector (14) from a plurality of areas (A-D) of the test sheet. The sensed magnetic effects are compared from two different groups (B, D) of one or more of the areas; and the test sheet is classified as unacceptable if the relationship between the magnetic effects of the compared groups is not within limits previously determined as acceptable for the relationship between the magnetic effects of the same pair of groups of areas of the reference sheet.

Abstract: This invention provides for the non-destructive testing of a longitudinally displaceable ferromagnetic body for structural faults. This device sends a stationary magnetic field transversely across the body of the test piece. Changes in this magnetic field, due to structural defects of the test piece, are detected by the magnetic field detectors which are placed on the periphery of the test piece in alignment with the stationary magnetic field. The changes detected by the magnetic field detectors must be processed in order to be useful. This processing requires compensation for the varying strength of the magnetic field at the surface of each different magnetic field detector, compensation for unequal sensitivities of each individual magnetic field detector in detecting structural defects, and also compensating for the tolerances of circuits which are used in the previous compensations.

Abstract: Equipment to measure flaws in an object using an eddy current. According to this equipment, attention is given to a relation between the amplitude of signal detected when the eddy current probe is scanning and the position of the eddy current probe, and the size of flaw is measured based upon the fact that scan distance of probe in which flaw detection signals of greater than a predetermined threshold value are obtained, is subject to change depending upon the depth of penetration of eddy current.

Abstract: A method and apparatus for detecting the presence of magnetic ink within a package without opening the package or altering its contents. The method and apparatus detects items marked with magnetic ink, such as bank checks, in an unopened package independently of the item's location within the package and despite the presence of such magnetic clutter as staples and paper clips. The apparatus and method is designed to eliminate magnetic noise from the equipment itself and from the environment.

Abstract: A tubing trip tool for use in determining the extent of defects in tubular sections of a continuous tubing string used in subterranean oil and gas wells is disclosed. The tubing trip tool is mounted on a surface rig surrounding the tubing string and defects are magnetically detected during tubing removal from the well bore. The tool comprises a segmented expandable detector head containing elements for measuring average wall thickness and local and axially extending defects. The expandable head is spring loaded and pneumatically actuated. Coupling detectors are located on the ends of the head to count tubing sections and to detect the presence of obstructions on the tubing to prevent damage to the detector head.

Abstract: Method and apparatus for nondestructive inspection of oilfield piping includes first and second elongated nonmagnetic housing members having an elongated magnetic core therebetween. A first coil assembly for emitting a unidirectional flux field is coupled to the core member. Detector shoes are coupled to said core member proximate the first coil assembly and are based into contact with the interior surface of the piping. Second and third coil assemblies are coupled to the core member for emitting unidirectional fields in opposition to the field emitted from the first coil assembly.

Abstract: A method and apparatus for determining the extent of defects in ferromagnetic tubular elements comprising a continuous string for use in an oil or gas well is disclosed. The tubing trip tool measures tubing average wall thickness, local defects, such as corrosion pitting, and axial defects, such as sucker rod wear during removal of the tubing from the well. Tubing velocity is also measured, and couplings between tubing sections are detected and counted, in order to specify the axial location of defects on each tube, and also provide a profile of the condition of the overall string. A saturating magnetic field and a fluctuating magnetic field are applied to the tubing and the magnitude of the induced fields and the changes are measured to quantify defects in the tubing.

Abstract: A method and apparatus for determining the extent of defects in ferrogmagnetic elements, such as tubing comprising a continuous string for use in an oil or gas well is disclosed. The tubing trip tool measures the depth of local defects, such as corrosion pitting, during removal of the tubing from the well. Tubing velocity is also measured, and couplings between tubing sections are detected and counted, in order to specify the axial location of defects on each tube, and also provide a profile of the condition of the overall string. A saturating magnetic field is applied to the tubing and a comparison of two derivatives of flux leakage is made to quantify defects in the tubing.

Abstract: A method for testing an electrically conductive test object with regard to an imperfection therein uses a transducer supplied with drive signals of two different frequencies so that via inductive coupling between the transducer and the test object, signal information is received from the transducer which can be processed to indicate the presence of an imperfection. The invention is characterized in that the transducer is designed and supplied with drive signals in such wise that the effect of the different depth penetration of currents induced in the test object at the two frequencies can be compensated for.

Abstract: A method and apparatus for magnetically measuring the amount and location of magnetite (Fe.sub.3 O.sub.4) debris adhering to the exterior of heat exchanger tubes which can be made of Inconel. A probe containing one or more coils of electrical wire is used to traverse the interior of the subject tube. One of the probe coils is energized with direct electric current to magnetize the magnetite coating on the tube exterior as the probe traverses the tube. The thickness of the magnetized coating is deduced from measurement of the voltage generated across a second coil of wire on a probe which traverses the tube at constant velocity.

Abstract: Defect detection in testing pipes, particularly in heat exchangers of nuclear reactors by means of eddy current in which interference influences due to the design are suppressed by measurements with different frequencies and the frequencies interlinked to give a signal only if there is a defect indication.

Abstract: In the testing of electrically conductive test objects for the presence of flaws and the like using a transducer to generate electrical currents in the test object and to inductively sense such currents, drive signals of at least two different frequencies are use for the transducer and the different frequency component induced signals picked up by the transducer are processed in such ways that a testing process is obtained that has low LO (lift-off) dependence. The characterizing feature of the invention is that the signal processing is undertaken as a function of at least one variable which may be the lift-off distance.

Abstract: A combined magnetic sensor and actuator device for applying a magnetic force to a magnetizable body and for sensing the distance between the device and the magnetizable body. The device includes a magnetizable pole piece separated from the magnetizable body by gaps. The pole piece, gaps, and magnetizable body form a magnetic circuit with the gaps preferably being the major reluctance of the circuit. Separate means are provided for generating a relatively large time-varying magnetic actuating flux in the magnetic circuit and for generating a relatively small time-varying magnetic sensing flux in the magnetic circuit. Each magnetic flux follows a flux path, such that the two flux paths have at least a portion in common. Detection means measure the relatively small magnetic flux in the magnetic circuit, and thereby measure the distance between the device and the magnetizable body.

Abstract: A system includes an oscillator for exciting an eddy current coil at one or more frequencies, detector circuitry for processing coil output signals and a balance and control calibration unit connected with the oscillator and the detector circuitry for so operating upon coil output signals in the detector circuitry as to force operation of the detector circuitry into a prescribed subrange of its overall operation range, and to effect correction of nonlinearity parameters bearing on system accuracy. The system is multi-frequency in makeup, the coil being excited at plural frequencies and the detector circuitry including separate detector channels corresponding respectively with the coil excitation frequencies. The coil output signals can be monitored in the detector channels for the occurrence in one or more of the channels of peaking of the coil output signal to a given magnitude above a threshold level.

Abstract: The invention relates to a multiple coil eddy current probe equipped with a coil balancing device. This probe comprises several coils forming an eddy current probe supplying in each case one signal. Each coil is regulated to overvoltage by a regulating unit, which acts on the frequency of the high frequency oscillator supplying each coil. Application is to the automatic positioning of a welding torch and to the inspection of the weld.

Abstract: An electromagnetic inspection system for detecting, in-situ, the location and character of flaws in a relatively long, thick-walled small bore tube of a heat exchanger wherein a resultant magnetic field is induced in the wall of the tube of selected direction and magnitude by the vector addition of the magnetic fields produced by two magnetic field generators, at least one of which is carried by a scanner for traversing the tube and having an array of sensors mounted in close proximity to the wall of the tube generating a unique signal as the scanner passes a flaw in the tube.

Abstract: During so-called eddy current testing by way of several frequencies, it is often difficult efficiently to separate and detect superficially located defects from the disturbances caused by a varying distance from the transducer to the object under test. This is due to the fact that the fault direction and the lift-off direction normally differ from each other to too small an extent and the fact that the signal processing, which is necessary for the suppression of the lift-off dependence, also suppresses the fault signal. This problem can be eliminated completely or partially by means of the present invention, which comprises selecting the used frequencies in such a way that, in relation to the depths of cracks occurring, they fulfill certain conditions, one of which being that at least one frequency is relatively insensitive to the defect in question. This effect is achieved by selecting such a high frequency that its depth of current penetration is limited seen in relation to the depth of the defect.

Abstract: In a method of casting a metal melt from a metallurgical vessel, in which the metal melt is covered by a slag layer, into another metallurgical vessel, the metal melt is guided through a casting tube, which covers the casting jet between the vessels. In order to reliably ascertain the passage of slag through the casting tube with very little expenditure, magnetic fields are induced in the casting jet by two coils peripherally surrounding the casting tube. The difference of the inductivity variations caused in the two coils by the eddy currents produced by the magnetic fields in the casting jet is continuously measured and compared to a least one limit value and casting is interrupted in case the limit has been exceeded or fallen short of.

Abstract: Methods and apparatus for producing alternating electromagnetic fields for use in theft detection and surveillance systems include individual series-type electrical drive circuits for separately energizing each of the field-generating coil windings with maximum efficiency and control, and transformer-type coupling between the different field-generating coil drive circuits to effect balanced and equalized current flow in the different coil windings and thereby enhance both drive efficiency and field uniformity. Additionally, a current-sense winding provides for monitoring of field coil current flow, and for feedback control of the magnitude and/or frequency of the coil excitation drive. In a preferred embodiment, the fields are generated by at least two coils on each opposite side of the field, and the coils are separately energized through separate but commonly-coupled drive circuits.

Abstract: A method and apparatus for producing a display of eddy current test data from signals produced by an eddy current detector which is supplied with an alternating current signal composed of alternating currents at several different frequencies, the detector signals including a respective signal associated with each current frequency and varying in amplitude and in phase relative to its associated alternating current as the detector is displaced relative to a test body.

Abstract: A tubing trip tool for use in determining the extent of defects in tubular sections of a continuous tubing string used in subterranean oil and gas wells is disclosed. The tubing trip tool is mounted on a surface rig surrounding the tubing string and defects are magnetically detected during tubing removal from the well bore. The tool comprises a segmented expandable detector head containing elements for measuring average wall thickness and local and axially extending defects. The expandable head is spring loaded and pneumatically actuated. Coupling detectors are located on the ends of the head to count tubing sections and to detect the presence of obstructions on the tubing to prevent damage to the detector head.

Abstract: A method and apparatus for determining the extent of defects in ferromagnetic tubular elements comprising a continuous string for use in an oil or gas well is disclosed. The tubing trip tool measures tubing average wall thickness, local defects, and axial defects, such as sucker rod wear during removal of the tubing from the well. Tubing velocity is also measured. A saturating magnetic field and a fluctuating magnetic field are applied to the tubing. Signals corresponding to changes in the induced fields are divided into channels for complete circumferential coverage. Phase shift is then determined by a zero crossing technique to quantify axial defects, such as sucker rod wear.

Abstract: An inductive proximity switch including an oscillator a circuit of which has an oscillator magnetic core adapted to be influenced by a trip device for the purpose of actuating an electronic switch. The proximity switch is adapted to detect an approach of the trip device, of a permeable material, through a nonferromagnetic wall so as to enable a triggering of the switching process. A sensor field formed of a magnetic field of a permanent magnet or an electromagnetic, with the proximity switch having a saturation-sensitive magnetization area or region in which the magnetic fluxes of the permanent magnet or electromagnet and oscillator core heterodyne. The inductive proximity switch is adapted to be operated in saturation by a reduction in a magnetic resistance produced by introducing the permeable trip device into the sensor field, damping the oscillator circuit and causing a switching of the electronic switch.

Abstract: The apparatus of the invention comprises at least two transmitter coils (3) and a respective number of receiver coils (4). The transmitter coils (3) and the respective receiver coils (4) are placed in a crosswise position with respect to each other so that the primary fields created by the transmitter coils (3) are divergent.

Abstract: A technique for detecting flaws in a metal product which uses a combined eddy current and magnetic flaw detection technique. A first magnetic field having flux lines parallel to the surface of the test object and a second field having flux lines perpendicular to the surface of the test object are generated. The resultant magnetic field is affected by faults in the metal test object. A magnetic field detector adjacent the test object detects the resultant magnetic field and produces an output signal which is processed by a circuit to provide a pair of component signals representing the portion of the output signal corresponding to the individual components of the resultant magnetic field, thus providing an indication of the type of faults that are present in the test object.

Abstract: A method for analyzing the anisotropic properties of ferromagnetic steels by use of Barkhausen noise simultaneously sensed in a plurality of directions without contact with the steel specimen.

Abstract: A uniform eddy current is produced in a part and the field is probed in several orthogonal positions by two coils to yield, for each position and for each coil, a signal that manifests the phase difference between the field and the signal producing the current and a signal that manifests the relative strength of the field.

Abstract: An apparatus and method for selectively orientating a ferrimagnetic body along a selected temperature invariant crystallographic axis includes a first pair of coils disposed for producing a magnetic field along a first direction, and a second pair of coils disposed within the first pair of coils for producing a magnetic field along a second direction, the direction of such second field being displaced at a predetermined angle .theta., with respect to the direction of said first field. A platform upon which the ferrimagnetic body is supported is disposed at an intersection of such fields and has a surface disposed at a predetermined direction with respect to a horizontal plane. A series of alternate pulsed magnetic fields is generated in turn by each pair of coils and the body rotates in response to each one of such fields. After pulsed fields have been generated, the so-called "easy axis" of the crystallographic structure of the body is aligned with the axis of the coils.

Abstract: A method and an instrument for testing materials using the eddy current principle, at different frequencies depending on the output quantity of a coil system, compensation values are generated being typical of interfering factors. These compensation values are subsequently superposed on the output values in order to remove the influence of the interfering factors on the measuring. In this manner with a favorable setting of the working point of an amplifier charged with the output values, an overmodulation is avoided and the dynamic range of the amplifier optimally utilized. Therefore good measuring results can be obtained even if the percentage of the interfering signals as compared to the signal controlling the detecting of defects, is relatively high.

Abstract: A device for detecting metallic objects in a material flow, comprising as sensing member a scanning coil consisting of a primary coil and, for example, two oppositely connected secondary windings arranged symmetrically around the primary winding so that the voltage induced in the secondary winding becomes zero as long as no conducting objects appear in the scanning area. To eliminate irrelevant signals from conducting objects in the vicinity of the scanning coil but not in the material flow or from weakly conducting objects in the material flow, the primary winding is fed with at least two alternating currents having different frequencies. The corresponding secondary voltages are rectified in phase-controlled rectifiers, and in a calculating circuit the difference is derived between the output of one rectifier voltage and the output of another rectifier multiplied by a constant factor which includes the relation between the frequencies of the corresponding alternating voltages.

Abstract: An apparatus for non-destructively testing ferromagnetic bodies to determine structural irregularities by means of magnetization of the respective object, comprises detectors for magnetic fields arranged close to the surface of the body, by which detectors any changes of behaviour of the magnetic field caused by structural irregularities can be determined. At least two stationary magnetic fields are arranged one after the other on the body that can be displaced in its longitudinal direction, the center axes of these magnetic fields being inclined toward each other at an angle. This angle is determined by the number of magnetic fields and is equal to the quotient from one hundred and eighty degrees and the number of magnetic fields. In the space between the poles of the magnetic field generators, the magnetic field detectors are stationarily arranged and in rows, each extending over a zone being equal to a body section to be tested, vertically to the shifting direction of the body.

Abstract: An ignition system transducer comprising a ferro-magnetic disc mounted on a distributor shaft and having four pairs of protuberances, the spacing of the two protuberances of each pair corresponding to the maximum and minimum spark advance positions. Rotation of the protuberances about the distributor shaft is detected by a first variable reluctance pick-up mounted in a fixed position, and by a second variable reluctance pickup movable in accordance with an engine load sensor. These two pick-ups in conjunction with signal generators produced a fixed phase waveform, and a variable phase waveform which varies in accordance with engine load.

Abstract: Supply circuit for an eddy current probe with two windings, comprising two parallel supply channels connected to the probe by a cable, each channel incorporating a reference winding and an amplification circuit having an input and an output, one of the ends of the reference winding being connected to the output of the amplification circuit and the other to the input of said circuit via a negative feedback resistor, wherein the amplification circuit of each channel comprises a first amplifier having an input constituting an input of the amplification circuit and an output connected to an absolute measurement access, a first resistor, whereof one end is connected to the output of the first amplifier, a second amplifier having an input connected to the other end of the first resistor and an output constituting the output of the amplification circuit, and a second resistor connected between the output and the input of the second amplifier.