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Showing papers on "Magnetic circuit published in 1974"


Journal ArticleDOI
TL;DR: In this article, an electromaanetic circuit model of three ohase transformers capable of includin ferromacanetic non-linear effects including a hvsteresis, suitable for anv number of windinas with a variety of interphase connections and applicable to any magnetic circuit confiauration.
Abstract: This paper describes an electromaanetic circuit model of three ohase transformers capable of includina ferromacanetic non-linear effects includinga hvsteresis, suitable for anv number of windinas with a variety of interphase connections and applicable to any magnetic circuit confiauration. In particular, three of the common types of magnetic circuits are considered i. e. (1) bank of sinqle phase cores (2) three limbb core and (3) five limmb core. The model is applied to tne solution of the inrush currents in a variety of situations and the results are shown to compare favourably with experimental results.

45 citations


Patent
07 Feb 1974
TL;DR: In this article, an electronic circuit breaker for use in A.C. power circuits is provided with means for varying the overload current response level at which the circuit breaker removes power from the load.
Abstract: An electronic circuit breaker for use in A.C. power circuits is provided with means for varying the overload current response level at which the circuit breaker removes power from the load. The load current is conducted through a primary winding of a transformer, and a circuit breaker switch is responsive to voltage across the secondary winding of the transformer exceeding a predetermined magnitude corresponding to the selected overload current level to remove power from the load. A lockout circuit prevents reapplication of power other than by a manual reset switch. The overload current response level corresponding to the predetermined magnitude of the voltage across the secondary winding is variably selected by varying the inductive coupling between the primary winding and the secondary winding. The primary winding is wound about one leg of a magnetic core and the secondary winding is wound about another leg of the magnetic core. In one embodiment, the inductive coupling is varied by altering a reluctance gap between the first and second legs of the core. In another embodiment, the inductive coupling is varied by changing the effective reluctance of a third leg of the core in parallel magnetic circuit with the first and second legs of the core. Both means for varying the gap and means for varying the effective reluctance of a third leg of the core may be combined to provide different ranges of selectibility. The overload current response level may also be adjusted by varying a reluctance gap in the first leg of the transformer core.

42 citations


Patent
Ajit V. Sabnis1
14 Jan 1974
TL;DR: In this article, a magnetic bearing assembly for levitating a movable member in either rotation or linear translation comprises a three-loop magnetic circuit in which permanent magnets establish a quiescent or fixed flux across four magnetic gaps arranged to provide restoring forces transverse to the direction of gap flux; the resulting inherent instability in the longitudinal direction is overcome by an electric feedback control system which supplies control currents in a pair of coils associated with the magnetic gaps for modulating the permanent magnetic flux therein and thereby provides stabilizing longitudinal magnetic forces.
Abstract: A magnetic bearing assembly for levitating a movable member in either rotation or linear translation comprises a three-loop magnetic circuit in which permanent magnets establish a quiescent or fixed flux across four magnetic gaps arranged to provide restoring forces transverse to the direction of gap flux; the resulting inherent instability in the longitudinal direction is overcome by an electric feedback control system which supplies control currents in a pair of coils associated with the magnetic gaps for modulating the permanent magnetic flux therein and thereby provides stabilizing longitudinal magnetic forces. The configuration may be generalized by simple extension to a (2n + 1) loop magnetic circuit with (n + 1) coils.

41 citations


Patent
21 Oct 1974
TL;DR: In this paper, a displacement transducer with a movable magnetic core and a fixed magnetic core oppositely arranged so as to have predetermined gaps or clearances in at least two places is described.
Abstract: A displacement - electricity transducer contains a movable magnetic core and a fixed magnetic core oppositely arranged so as to have predetermined gaps or clearances in at least two places. Two substantially closed magnetic circuits, each including one of the gaps, are formed by both magnetic cores, so that when the movable magnetic core is displaced relative to the fixed magnetic core while maintaining a predetermined spacing therefrom, the substantially opposite area of the gap portion in one of the magnetic circuits increases, while the substantially opposite area of the gap portion in the other magnetic circuit decreases. A first coil is wound around a part common to the two magnetic circuits, while a second coil is wound around a part not common thereto. By applying an alternating current to the first coil, an output responsive to the displacement of the movable magnetic core is derived from the second coil.

40 citations


Journal ArticleDOI
TL;DR: In this article, two types of parametric transformers referred to as the "chain type" and "cross-winding type" are described, which are related in the sense that they have identical magnetic circuits, but they differ in their windings arrangement.
Abstract: The paper describes the two types of parametric transformers referred to as the "chain type" and "cross-winding type". They are related in the sense that they have identical magnetic circuits, but they differ in their windings arrangement. These transformers utilize the principle of parametric oscillations to develop an output voltage and transfer energy from the input circuit to the output circuit without direct mutual coupling. The two types belong to two different categories in which the MMF's of the input and output circuits act either at right angle to one another (orthogonal) or along the same line (co-linear) in the common regions. The operation of each type is described by two simultaneous nonlinear differential equations and the magnetization characteristics which relate the MMF's and fluxes in the input and output sides of the transformer. The experimental results on a small unit show that the cross-winding type is more efficient and has a higher output to weight ratio than the chain type.

27 citations


Patent
22 Mar 1974
TL;DR: In this article, the magnetic field is generated by thin metal spacers, which are used to collapse the respective magnetic fields of the flexible magnetic components to increase the flux density at the resultant poles and to orient the permanent magnetic fields in the magnetic circuit of the motor.
Abstract: Compound magnets for motor stators have a laminated structure that includes thin, flexible magnets made from permanently magnetizable particles, such as barium ferrite, that are embedded in a flexible matrix, such as rubber Various laminated arrangements are contemplated for producing more intense magnetic fields and thin metal spacers are used in most laminated structures to collapse the respective fields of the flexible magnetic components to increase the flux density at the resultant poles and to orient the permanent magnetic fields in the magnetic circuit of the motor With edge polarized magnetic layers, an outwardly offset side edge arrangement for the spacers is used to among other things provide a pole piece for the motor as well as intensify the field With facially polarized magnetic layers, the edges of the spacers are inwardly offset in the laminated structure Various arrangements are shown to simplify construction costs and to establish focus field paths

24 citations


Patent
05 Jun 1974
TL;DR: An angular position transducer as mentioned in this paper consists of a permanent magnet with closely spaced pole faces and a ferromagnetic core magnetically connected to the magnet and carrying a coil, which cooperates with a toothed wheel.
Abstract: An electromagnetic angular speed or angular position transducer comprises a permanent magnet with closely spaced pole faces and a ferromagnetic core magnetically connected to the magnet and carrying a coil. The transducer cooperates with a toothed wheel the teeth of which are moved past the pole faces and the core in such a way that two magnetic circuits are completed by the teeth, one passing through the teeth and the two pole faces, and the other passing through the teeth and the core. The two magnetic circuits are in parallel with one another and the reluctance of each changes as the teeth move therepast. The circuit through the two pole faces ensures that the magnet is operated with a high leakage flux so that small changes in the reluctance cause relatively large flux changes and thus a relatively high electromotive force in the coil wound about the core.

24 citations


Patent
08 Oct 1974
TL;DR: A magnetic bearing assembly for levitating a movable member in either rotation or linear translation comprises a three-loop magnetic circuit in which permanent magnets establish a quiescent or fixed flux across four magnetic gaps arranged to provide restoring forces transverse to the direction of gap flux; the resulting inherent instablity in the longitudinal direction is overcome by an electric feedback control system which supplies control currents in a pair of coils associated with the magnetic gaps for modulating the permanent magnetic flux therein and thereby provides stabilizing longitudinal magnetic forces as mentioned in this paper.
Abstract: A magnetic bearing assembly for levitating a movable member in either rotation or linear translation comprises a three-loop magnetic circuit in which permanent magnets establish a quiescent or fixed flux across four magnetic gaps arranged to provide restoring forces transverse to the direction of gap flux; the resulting inherent instablity in the longitudinal direction is overcome by an electric feedback control system which supplies control currents in a pair of coils associated with the magnetic gaps for modulating the permanent magnetic flux therein and thereby provides stabilizing longitudinal magnetic forces The magnetic core, permanent magnet, and coil configuration permits a modification to the electric feedback control system whereby the transverse restoring forces may be controlled to provide transverse stiffness augmentation

23 citations


Patent
Harry L Maddox1
29 Apr 1974
TL;DR: In this article, a field of magnetic flux generated by an electrical induction coil having applied to a constant amplitude, alternating voltage wave, the half cycles of which alternate in polarity and successively and incrementally increase in frequency.
Abstract: To demagnetize a magnetic material, the material is positioned within a field of magnetic flux generated by an electrical induction coil having applied thereto a constant amplitude, alternating voltage wave, the half cycles of which alternate in polarity and successively and incrementally increase in frequency. The flux alternates in polarity with the polarity of the voltage wave, and the magnitude, or strength, of the flux is in accordance with the current flow through the coil, or is inversely in accordance with the impedance of the coil. As the frequency of the voltage wave incrementally increases from an initial low value to a higher value, the magnitude of the magnetic flux incrementally decreases from an initial high value to a lower value, and the alternating polarity, decreasing magnitude magnetic flux effectively demagnetizes the magnetic material. The invention finds utility in demagnetizing magnetic material cores of magnetic operating coils.

23 citations


Journal ArticleDOI
TL;DR: In this article, the induced magnetic force on a unit charge moving around a circuit is shown to do work on the unit charge, and it is shown that magnetic forces can never do work.
Abstract: Consider a conducting circuit moving with velocity v through a constant magnetic field B. The induced emf is given by ∫ v × B · dl where the integral is taken once around the circuit. Some texts refer to v × B as the force the magnetic field exerts on a unit charge moving around the circuit. This is incorrect as magnetic forces can never do work. The force the conductor exerts on an electron is shown to do this work.

21 citations


Patent
02 Jul 1974
TL;DR: In this article, a gap in a magnetic circuit of an iron core inductor is provided, and a plurality of small size permanent magnet pieces are placed in the gap, with the magnetizing directions thereof being provided in side-by-side relation.
Abstract: There is provided a gap in a magnetic circuit of an iron core inductor, and a plurality of small size permanent magnet pieces are placed in the aforesaid gap, with the magnetizing directions thereof being provided in side-by-side relation. If the magnetizing direction of the permanent magnet pieces is opposed to that of a D.C. magnetic field which is produced in a magnetic circuit, yet in case D.C. and A.C. overlapping currents flow through the inductor, then the D.C. magnetic field produced in the magnetic circuit will be off-set thereby, providing a high inductance value, while eddy currents produced within the permanent magnet pieces due to the A.C. magnetic field will be reduced to a considerable amount, because the permanent magnet pieces are small in size, thus minimizing the heat to be produced in the permanent magnet pieces.

Patent
19 Dec 1974
TL;DR: In this paper, a variable reluctance motor acts as its own position sensor, sensing the EMF induced in the motor winding while the motor is rotating and while the switching device output is nonconductive.
Abstract: A control circuit for a variable reluctance motor. A solid state switching device has its output circuit connected in series with the electrical winding of the motor. Circuit means are coupled to the motor winding and to the control electrode of the solid state switching device. The circuit means senses the EMF induced in the motor winding while the motor is rotating and while the switching device output is nonconductive. The switching device output circuit is rendered conductive when the EMF attains a predetermined level. The EMF induced in the winding of the rotating motor results from residual magnetism in the magnetic circuit of the motor or from such residual magnetism in addition to a bias current supplied to the motor winding. The variable reluctance motor acts as its own position sensor.

Journal ArticleDOI
TL;DR: In this article, a dynamical model is described which permits calculation of the excitation current I as a function of time in a laminated grain-oriented (G-O) steel transformer core.
Abstract: A dynamical model is described which permits calculation of the excitation current I as a function of time in a laminated grain-oriented (G-O) steel transformer core. The independent variable is the magnetic flux density or, equivalently, the coil voltage less the IR drop associated with the resistance R of the windings. Recent observations on flux reversal mechanisms in GO steel indicate that, in the range of magnetic field intensities typically present in transformer cores, the important reversal processes are the motion of long 180° domain walls continuous across grain boundaries and the motion of 90° walls within individual grains. These processes are represented in the model by two subcircuits connected in series. Each subcircuit consists of an inductive element in parallel with a linear resistor which accounts for the eddy current losses accompanying the flux change. The properties of each inductive element (flux vs. current) reflect the two wall motion mechanisms, respectively, in the limit of zero frequency. This model is capable of faithfully simulating minor loop behavior as well as the response to complex waveforms; e.g., the superposition of two or more frequencies. The circuit equations are solved, and some results of computer calculations using a program that implements this model are presented.

Journal ArticleDOI
TL;DR: In this paper, a dc-ac converter with a magnetic circuit has been proposed, which can be converted to alternating sine wave voltage with an optional frequency and even if the load current is increased, the values of the output voltage and its frequency are constant over a wide range.
Abstract: The following is a brief description of the operation principle and the circuit configuration of a new dc-ac converter proposed by the authors. This device is constructed by a magnetic circuit of unusual shape and has a number of excellent characteristics in the operation. Therefore dc voltage can be converted to alternating sine wave voltage with an optional frequency, and even if the load current is increased, the values of the output voltage and its frequency are constant over a wide range.

Patent
Drubig Horst1
19 Sep 1974
TL;DR: In this article, a rapid-action, current-limiting automatic switch with a magnetic tripping device with an iron circuit of small geometric dimensions and few ampere turns of the exciter coil required to make the armature acting on the latching mechanism respond is described.
Abstract: A rapid-action, current-limiting automatic switch with a magnetic tripping device with an iron circuit of small geometric dimensions and few ampere turns of the exciter coil required to make the armature acting on the latching mechanism respond, comprises an additional armature which acts on a movable contact member and is arranged in a magnetic circuit which is in common with the first armature. The tripping device contains a magnetic core, an exciter coil and a magnetic shunt across the air gap of the additional armature.

Journal ArticleDOI
TL;DR: In this article, the authors discuss both the design of the field coils as well as the drive circuitry which is required to produce transient-free starting and stopping of the rotating in-plane magnetic field.
Abstract: This paper is concerned with the generation of the rotating in-plane magnetic field which is required by field-accessed bubble devices We discuss both the design of the field coils as well as the drive circuitry which is required to produce transient-free starting and stopping of the rotating field Details of coil design are given for a multichip 1 Mbit module and a small (025 sq cm) single chip memory The circuitry required to drive these coils at 1OO kHz and 1 MHz is discussed in detail

Journal ArticleDOI
TL;DR: In this article, the authors present a steady state analysis of a magnetic frequency tripler without primary line filters and evaluate the effect of different iron magnetization characteristics upon, the performance of the device.
Abstract: The authors present a steady state analysis of a magnetic frequency tripler without primary line filters. The purpose of the paper is to evaluate the effect of different iron magnetization characteristics upon, the performance of the device. The results of the analysis can be used as guidelines in the selection of iron in the design of large frequency triplers.

Patent
22 Apr 1974
TL;DR: In this paper, a parallel-connected inductance and capacitor are connected in parallel with a normally closed switching device and in series with an electric power distribution system, and means are provided to produce a high arcing voltage in a pressurized liquid medium.
Abstract: A parallel-connected inductance and capacitor are connected in parallel with a normally closed switching device and in series with an electric power distribution system. The capacitive reactance of the capacitor is substantially higher than the inductive reactance of the inductor at the power line frequency. The normally closed switching device is opened in response to a predetermined level of fault current in the system so that the reactor is inserted into the power distribution system to effect limitation of current level. The switching device is springbiased to a closed position and the current path through the switch contacts forms a blow-open magnetic circuit, whereby the contacts are blown open against the closing force of the biasing springs when the predetermined current magnitude is reached. Means are provided to produce a high arcing voltage in a pressurized liquid medium.

Journal ArticleDOI
P. Unterweger1
TL;DR: In this paper, the effect of material inhomogeneity on the structure of the matrices which numerically model the physical problem and the effect that structure on the convergence of iterative schemes for the solution of the simultaneous equations is examined.
Abstract: The determination of two-dimensional magnetic flux distributions in electrical machines and apparatus such as motors, generators and transformers usually involves regions of air, other non-magnetic materials and magnetic materials. The effect of such material inhomogeneity on the structure of the matrices which numerically model the physical problem, and the effect of that structure on the convergence of iterative schemes for the solution of the simultaneous equations, is examined in this paper. A block iterative procedure which takes advantage of the matrix structure resulting from the inhomogeneity is described.

Journal ArticleDOI
TL;DR: In this paper, the magnetic field distribution inside a ferromagnetic bar rotating in a magnetic flux set up by a two pole structure is compared with the results obtained by linearly superposing direct and generated cross axis fields at a particular speed of rotation.
Abstract: Finite element analysis techniques are being used widely to predict the field distribution in magnetic structures having complex boundary geometries and nonlinear B-H characteristics. In situations where two or more magnetic fields interact, care must be taken to ensure that the principle of superposition applies before adding the fields directly. The magnetic Reynolds number can be used to predict linearity, and in those cases where superposition is invalid, the finite element algorithm must be applied repeatedly in an incremental fashion to ensure accuracy. This procedure is illustrated by considering the magnetic field distribution inside a ferromagnetic bar rotating in a magnetic flux set up by a two pole structure. Results obtained by linearly superposing direct and generated cross axis fields at a particular speed of rotation are compared with the magnetic field distributions resulting from repeated application of the finite element algorithm for incremental step changes in angular velocity. Comparison of the neutral axis angle/speed characteristic for the linear and incremental models shows a significant difference between the two approaches.

Journal ArticleDOI
TL;DR: In this paper, a theoretical model for the relationship among magnetic quantities describing the one- versus two-dimensional field in a permanent magnet was developed and applied to two problems: 1) determination of the direction of the residual magnetism and 2) finding the magnetic flux distribution in a magnet stabilized in air.
Abstract: A theoretical model for the relationship among magnetic quantities describing the one- versus two-dimensional field in a permanent magnet is developed. The model is applied to two problems: 1) determination of the direction of the residual magnetism and 2) finding the magnetic flux distribution in a magnet stabilized in air. The vector potential equation in a form unified for both problems is derived and transformed into a partial-difference algorithm. The numerical line-iteration procedure is described and used to solve these problems for a six-pole permanent magnet rotor of an electrical alternator.

Patent
15 Feb 1974
TL;DR: The two armatures of the electromagnetic relay are magnetically in series with the coil cores when the magnetic flux of their permanent magnets flows in the same direction as discussed by the authors, which simplifies mounting and securing of the relay, while the spreading motion of the construction part is practically eliminated.
Abstract: The two armatures of the electromagnetic relay contain permanent magnets and one or two coils. The armatures are magnetically in series with the coil cores when the magnetic flux of their permanent magnets flows in the same direction. Preferably the armatures are H-shaped, forming permanent magnets with parallel pole sheets, extending beyond the magnets in the plane at right angles to the rotary axis of the armature on both sides. The extending parts abut the pole shoes of the magnetic circuit. The pole shoes may be identical design. The design simplifies the mounting and securing of the relay, while the spreading motion of the construction part is practically eliminated.

Patent
19 Feb 1974
TL;DR: In this article, a high-speed advancing and printing head suitable for printing on thermally sensitive paper is provided by completion of a magnetic flux circuit formed in part by a flux carrying means that extends parallel to the travel of the head.
Abstract: A printer with a high-speed advancing and printing head suitable for printing on thermally sensitive paper, in response to computer commands. Good contact between the printing head and the paper is provided by completion of a magnetic flux circuit formed in part by a flux-carrying means that extends parallel to the travel of the head. This magnetic circuit permits the use of a small, low-inertia printing assembly and thus facilitates high speed movement from one print position to the next. The use of the same magnetic flux circuit for braking of the printing head and carriage by sliding friction upon the paper prevents overshoot and oscillation when the head approaches a new print position.

Journal ArticleDOI
TL;DR: In this article, a technique for applying a burden to a current transferer being calibrated in which the power and reactive volt-amperes are returned to the supply rather than being absorbed in a passive network is presented.
Abstract: A technique is presented for applying a burden to a current transf ormer being calibrated in which the power and reactive volt-amperes are returned to the supply rather than being absorbed in a passive network. The transfer is accomplished either by the magnetic shield if a compensated current comparator is used as the ratio standard, or by an auxiliary current transformer in circuits which employ other standards. Electronic circuitry provides the necessary volt-amperes for exciting the magnetic shield of the current comparator or the auxiliary current transformer, and the current transformer being calibrated, to produce the desired power transfer. The circuit requirements are analyzed and equipment which can develop burdens with both in-phase and quadrature components up to 10 ohms, at secondary currents up to 10 amperes, and for a 0.6 percent accuracy class current transformer or better, is described. The burden setting, which is independent of current, may be made by in-phase and quadrature dials to an accuracy of about one percent of full scale.

Journal ArticleDOI
TL;DR: A mathematical model is developed to represent the dynamic behavior of multiple-stack variable-reluctance step motors and is used to simulate the dynamic response of the step motor under several operating modes.

Patent
22 Apr 1974
TL;DR: In this article, a parallel-connected inductance and capacitor are connected in parallel with a normally closed switching device and in series with an electric power distribution system, and means are provided to produce a high arcing voltage in a pressurized liquid medium.
Abstract: A parallel-connected inductance and capacitor are connected in parallel with a normally closed switching device and in series with an electric power distribution system. The capacitive reactance of the capacitor is substantially higher than the inductive reactance of the inductor at the power line frequency. The normally closed switching device is opened in response to a predetermined level of fault current in the system so that the reactor is inserted into the power distribution system to effect limitation of current level. The switching device is springbiased to a closed position and the current path through the switch contacts forms a blow-open magnetic circuit, whereby the contacts are blown open against the closing force of the biasing springs when the predetermined current magnitude is reached. Means are provided to produce a high arcing voltage in a pressurized liquid medium.

Journal ArticleDOI
TL;DR: In this paper, an experimental study of the magnetic field of a rectilinear system with a helical magnetic axis was performed using a BESM-6 computer, and the results showed that the scalar potential and the components of magnetic field strength inside a straight solenoid with a dense winding were derived.
Abstract: A study is made of the magnetic field of a solenoid formed by current-carrying loops, the plane of which is perpendicular to a helical line. Analytical expressions are derived for the scalar potential and the components of the magnetic field strength inside a straight solenoid with a dense winding. The toroidal case was investigated using a BESM-6 computer. The results are presented of an experimental study of the magnetic field of a rectilinear system with a helical magnetic axis. The paper discusses the possible design of a toroidal trap with the magnetic configuration in question.

Patent
20 Sep 1974
TL;DR: In this article, a flexible magnetic sheet reflective on one side, at least one magnetic circuit associated with one zone of the sheet for deforming the sheet by a modification of the magnetic flux in the magnetic circuit.
Abstract: The device comprises a flexible magnetic sheet reflective on one side, at least one magnetic circuit associated with at least one zone of the sheet for deforming the sheet by a modification of the magnetic flux in the magnetic circuit. Exciting means controlled by electrical data vary the magnetic flux so as to modify the reflecting conditions in said zone as a function of the electrical data. A unit for recording holographical optical memories is disclosed which employs this device.

Patent
G.F. Hughes1
14 Feb 1974
TL;DR: An improved linear magnetic actuator for use in a disk drive is described in this article, where a single turn shorted copper sleeve is plated onto the bore surface of the outer pole to minimize coil winding inductance.
Abstract: An improved linear magnetic actuator for use in a disk drive is described. High performance linear magnetic actuators are required in disk drive units to move read/write heads quickly and accurately from one track on a disk file to another. A powerful magnetic field for use in a magnetic actuator can be produced at relatively low cost by using flat slab magnets. An outer pole of square outside dimensions, and with a central circular bore, is used to transmit the magnetic lines of force from the flat magnets to the circular coil winding. A single turn shorted copper sleeve is plated onto the bore surface of the outer pole to minimize coil winding inductance. Four stationary center poles are positioned inside the movable coil assembly to provide a path for the magnetic lines of force to the ends of the magnetic actuator, from there to the bottom, side and top plates of the magnetic actuator, and finally back to the magnets to complete the magnetic circuit. Each center pole has a gap positioned at its approximate center so that under high current conditions saturation will occur at the gap of the center pole and not elsewhere in the magnetic actuator, thus preventing a distortion of the permanent magnetic field under high current conditions. The actuator is completely encased in flat steel plates to prevent the generation of stray magnetic fields.

Patent
20 Jun 1974
TL;DR: In this paper, a means for inducing an alternating magnetic field in the pipe being checked and a converter for converting a change in the magnetic field into an electric signal, is described.
Abstract: The apparatus according to the invention may be used in the oilrecovery and gas industry. The apparatus comprises means for inducing an alternating magnetic field in the pipe being checked and a converter for converting a change in the magnetic field into an electric signal, said means and said converter comprising two transformers mounted in a fixed position relative to each other and electrically coupled therebetween, and an electronic amplifier circuit with positive feedback formed by the transformer windings. The magnetic circuits of the both transformers are completed by the body of the pipe being checked. Upon the occurence of a discontinuity in the pipe being checked the magnetic flux values of the both transformers are changed so that the frequency of the output signal of the converter is also changed to obtain the information on the discontinuity of the metal of the pipe.