Abstract: A sensor comprises a magnetic flux divider and a winding. The magnetic flux divider comprises first and second magnetic paths through which magnetic flux flows. The magnetic flux divider divides the magnetic flux between the first and second magnetic paths as a function of a total amount of magnetic flux flowing through the magnetic flux divider. The winding produces a signal that is indicative of the total amount of magnetic flux flowing through the flux divider based on the operation of the flux divider. A method of sensing a parameter comprises providing a material with a permeability that varies in accordance with an amount of magnetic flux passing through the material; and acquiring information indicative of the permeability of the material, the permeability of the material being indicative of the parameter. The sensor and method can be utilized to acquire information pertinent to another parameter of ultimate interest, such as information pertaining to a current flow in an electrical circuit.

Abstract: A signal sensor is provided that includes a substrate, an input port formed on the substrate as a substantially linear conductive element, and a sensing port formed on the substrate adjacent to the input port. The sensing port is influenced by magnetic flux emanating from the input port such that a sense signal is generated in the sensing port. The sensing port can have as little as one sensing loop disposed on one side of the input port, but preferably includes at least one sensing loop on each side of the input port. The sensing loops on each side of the input port have an opposite sense of turn so that the sense signals in each loop are additive. A cross-over connector provides an electrical connection between the loops of the sensing port on opposite sides of the input port. The cross-over connector can be an underpass crossing below the input port or an overpass crossing above the input port. If desired, multiple sensing loops can be formed on each side of the input port.

Abstract: The invention relates to sensors for detecting a time-varying current by employing a plurality of coils with separate windings, disposed on separate toroidal cores, that are placed in close proximity of each other so that each coil responds independently to the same current. The current produces a time-varying magnetic field which in turn induces a plurality of voltages across the coils that can be combined to provide a resultant signal. Some embodiments of the invention employ coils with opposite windings to obtain signals with opposite phases and combine these signals through differential detection means to obtain a combined signal. One aspect of the invention relates to production of a wide-band or a selectable band-width sensor by preparing at least one coil to be dissimilar with respect to the others. In addition, the invention provides provisions for easy calibration of the sensor.

Abstract: A current sensor for an electrical device including a primary circuit in which a primary current flows. A magnetic circuit includes two identical U-shaped elements between which two studs for air gaps are located. A secondary circuit includes two similar coils of a cylindrical shape, mounted in series and encircling the air gaps, wherein these two coils are made in a continuous way with only one winding operation without any electrical connection between both coils. The winding of the primary circuit encircles one of the two coils. The sensor is designed so that it may be adapted to several nominal ranges of primary current by varying the number of passages of the primary circuit between coils, the number of windings of the secondary circuit, and the thickness of the air gaps.

Abstract: An inductive sensor for measuring a current in a conductor includes a generally annular winding (10) delimiting a passage for the conductor. The winding extends homogeneously over most of the circumference of a ring except for a section (20) where the winding has a structural discontinuity. The sensor includes at least one identical complementary winding (12). The windings (10, 12) are disposed coaxially and oriented so that the structural discontinuities of the windings are regularly distributed in the angular direction about the common axis of the windings. The windings are connected in series.

Abstract: A combined filter inductor and Hall current sensor for a double inverter motor drive is formed on a unitary magnetic core. The magnetic core has a suitable winding and air gap to comprise an inductor. The Hall effect sensor is situated in the airgap to detect a Hall effect voltage created by the magnetic flux in the air gap.

Abstract: A two-stage filter for a circuit protection device. The filter includes a first filtering device connected to receive an input signal v, the first filtering device generating an intermediate signal v1, wherein v 1 ⁢ ( k ) = ∑ n = 0 20 ⁢ h n · v ( k - n ) , where k is a sample index and h0 . . . h20 are first filter coefficients.

Abstract: A self powered parallel bar current sensor for generating a signal which accurately represents current in a power line includes a power coil core having an opening extending therethrough, and a current sensing element including two substantially parallel sensing bars and a sensing coil. The sensing coil is disposed between the sensing bars, and the sensing bars and sensing coil extend through the power coil core opening. A reference coil may also be situated between the sensing bars. The current sensor is connected in series in the power line.

Abstract: To perform broadband current detection. The current detector of the present invention comprises first terminal, which receives power source current; second terminal, which feeds output current to an outside current; third terminal, which outputs pilot current having a predefined relationship with this output current; and fourth terminal, which has reference potential; as well as first component connected between the first and second terminals; vertical connection of first and second baluns and coupled with the first component; grounded capacitive component at the connecting part of the first and second baluns; and grounded capacitive component at the output part of the second balun. The properties of the instrument for measuring impedance and the apparatus for measuring power are improved by using the current detector of the present invention.

Abstract: A fiscal electricity meter is described for measuring the energy supplied to a load. The load current flows through the primary winding of a transformer and induces an EMF indicative of the current flowing in the secondary winding. The secondary winding comprises a sense coil, arranged to couple more strongly to the primary, and a cancellation coil which have equal and opposite turns area products so as to provide a null response to extraneous magnetic fields. The coils are arranged so that their magnetic axis are co-located and aligned together so that they also provide a null response to extraneous magnetic fields having a field gradient.

Abstract: A fixed type current detector 20 of the present invention includes: a magnetic flux conversing fixed member 22, which is fixed along the periphery of a long conductor 21, and which converges the magnetic flux generated around the conductor 21 at the time of current sending to the conductor 21; and a magneto-electric conversion element 23, which is integrally provided to the magnetic flux conversing fixed member 22, and which detects the magnetic flux of the magnetic flux conversing fixed member 22, and in the magnetic flux conversing fixed member 22, a high permeability member 26 to converge the magnetic flux generated around the conductor 21, is laid.

Abstract: A system and method for measuring current flow through a conductor. The system includes a core having a general ring shape and a central opening. The core terminates at two ends that form a gap for receiving a Hall generator. The core is electrically coupled to a ground to prevent electrical noise on the conductor from being coupled to the Hall generator, thus permitting the direct use of the voltage signal output by the Hall generator.

Abstract: A current sensor assembly includes a plurality of symmetric current conductors that pass asymmetrically through an opening in a toroidal core having a sensing winding wound on the core. A blade is connected to each end of the conductors. Each of the conductors has the general shape of a “V” as viewed along a central axis passing through the opening in the toroidal core.

Abstract: The invention relates to sensors for detecting a time-varying current by employing a plurality of coils with separate windings, disposed on separate toroidal cores, that are placed in close proximity of each other so that each coil responds independently to the same current. The current produces a time-varying magnetic field which in turn induces a plurality of voltages across the coils that can be combined to provide a resultant signal. Some embodiments of the invention employ coils with opposite windings to obtain signals with opposite phases and combine these signals through differential detection means to obtain a combined signal. One aspect of the invention relates to production of a wide-band or a selectable band-width sensor by preparing at least one coil to be dissimilar with respect to the others. In addition, the invention provides provisions for easy calibration of the sensor.

Abstract: A current sensing cell, using PCB trace-loops, with the loops either contained in a single layer—horizontal loops, or in two separate layers—vertical loops interconnected by vias into a single two-pole inductive component. The horizontal loop-sensor cell will be used sideways, next to a planar conductor carrying a current parallel to the sensor traces, so as to collect the magnetic field lines on the side of the planar conductor, where they come closer together. The vertical loop sensor cell will be used on top or bottom of a planar conductor carrying current parallel to the cell, so as to collect the magnetic field lines going across the conductor, or between two parallel conductors, carrying parallel current of opposite sense, so as to collect the magnetic field lines that run in between the two conductors, summing up the two fields.

Abstract: The current to be measured (I) passes in a conductor (1) enclosed with a Rogowski coil (4) and in the proximity thereof are arranged several magnetic field detectors (3). The sum of the signals from these detectors, formed in the device (2), and the signal supplied by the Rogowski coil are applied to a signal processor (7). The processor (7) comprises, for example, a first order delay device connected to the Rogowski coil output and means for amplifying/reducing the signals followed by adding means, the delay time constant and the amplifying/reducing factors being selected so that the device output (8) signal is proportional to the intensity of the current to be measured in a very wide frequency range, i.e. from 0 to more than 10 MHz.

Abstract: There is provided a high frequency current detecting apparatus which can measure a high frequency current with high precision without cutting a circuit conductor. The high frequency current detecting apparatus includes a core having a hollow portion, a first conductive material inserted in the hollow portion of the core and having both ends respectively protruding from both ends of the hollow portion to form probe portions, a second conductive material electrically insulated from the first conductive material and inserted in the hollow portion of the core, a resistor for generating a voltage corresponding to a high frequency current flowing through the second conductive material while the first conductive material is made a primary coil and the second conductive material is made a secondary coil, and a shield provided around the core.

Abstract: For precise measurement of currents with high dynamic range, especially in gas-insulated voltage switchgear, a first signal of a first low-power measurement transformer and a second signal of a second nonmagnetic measurement transformer, operating, for example, according to the principle of a Rogowski coil or according to the Faraday effect, are compared. Depending on the degree of accuracy, one of the two signals is then selected and further processed. A deviation of the two signals from each other, especially their gradient, is referred to as comparison criterion. In a corresponding configuration, the two measurement transformers of different type are connected to a common processing component for signal processing.

Abstract: A current sensor assembly includes a plurality of symmetric current conductors that pass asymmetrically through the sensor assembly. A blade is connected to each end of the conductors. The conductors have a diagonal symmetry and have the general shape of a “V” as viewed from the top of the sensor assembly. The conductors pass through an opening located at the center of the current sensor.

Abstract: An ultra high frequency current probe is fabricated in the form of a surface mount device, designed to be a part of the circuit providing the current being observed.

Abstract: A Swain type direct current clamp-on sensor called MER2 is disclosed with improved magnetic error reduction from undesired magnetic fields generated by noise sources for accurate measurement of low currents. The sensor is comprised of a split core with non uniform magnetic structure designed to concentrate the magnetomotive force generated by the current being measured, and also by the switching magnetomotive force, through the windings near the lips of the sensor for better signal to noise ratio. The sensor core has non-uniform windings with greater ampere-turns near the lips than in other sectors of core remote from the lips so that the flux density of magnetic material in and near the lips is better switched. Furthermore, there is a provision for combining and adjusting current signals such that the noise is largely canceled. A method of making the direct current clamp-on MER2 is also disclosed.

Abstract: The invention concerns an electric current pick-up shoe compensating fluxes generated in a magnetic circuit (1) by respectively a primary current and a compensating current circulating in a secondary coil. Said secondary coil (5) comprises at its surface a layer of spires (12) forming a protective winding connected to a fixed electric potential (13), such that the effect of leakage capacities between the primary current conductor (4) and the secondary coil (5) winding (6) are practically eliminated.

Abstract: A circuit for measuring current flowing through a load driven by a pulse width modulation (PWM) circuit in response to PWM control signals, according to the present invention, includes a transformer having primary and secondary windings, with the primary winding connected to the load. Current cancellation circuitry coupled to the secondary winding is adapted to apply a cancellation current to the secondary winding and to provide an output which is indicative of a level of current through the load. An inverter coupled to the current cancellation circuitry output provides an inverted current cancellation circuitry output. First and second switches selectively couple one of the current cancellation circuitry output and the inverted current cancellation circuitry output to a load current output which provides an output indicative of the level of current through the load during both static and dynamic load current conditions.

Abstract: A Rogowski coil measures a current in a conductor and includes first and second loops electrically connected in series with each other. The first loop is wound with a substantially constant winding density in a first direction around a first core that has a substantially constant cross section. The second loop is wound with a substantially constant winding density in a second direction around a second core that has a substantially constant cross section. A direction of the first loop is substantially perpendicular to the normal of the first core cross section. Moreover, a direction of the second loop is substantially perpendicular to the normal of the second core cross section.

Abstract: A system for testing an arcing fault detection system in an electrical distribution network having a line conductor carrying an electrical signal between a power source and a load, the arcing fault detection system including a sensor coupled to the line conductor for monitoring the electrical signal and generating a sensor signal representing the electrical signal, the arcing fault detection system generating an arc-indicative signal in response to the sensor signal having characteristics indicative of an arcing fault. The testing system couples the sensor to a test line simultaneously with the line conductor and periodically produces a test signal on the test line. The sensor simultaneously monitors the test signal and the electrical signal and produces a sensor signal representing both the test signal and the electrical signal when the test signal is present on the test line.

Abstract: A method and device for measuring electric currents in n conductors (n≧2), the measurement of the currents being carried out with (n−1) magnetoresistive sensors. Reliable excess current and/or fault current protection is obtained.

Abstract: A device (16) for sensing current flowing in a generally flat plate structure (10) contains a magnetic head (18) and signal processing circuitry (20). The magnetic head (a) senses changes in current-induced magnetic flux as the head is positioned over the plate structure and (b) provides a head output signal. The signal processing circuitry processes the head output signal to produce a data signal indicative of how much current appears to flow in the plate structure below the head. A driving voltage, which typically varies in a periodic manner to produce a characteristic signature, is applied to a primary conductor in the plate structure. A location sensor, typically formed with a light source (100) and a light sensor (102), detects the position of the magnetic head relative to the plate structure. A gas-cushion mechanism (80-98) controls the height of the head above the plate structure.

Abstract: A conduit apparatus for an electrical enclosure that allows safe, convenient access for current measurement of circuits therein is provided. The conduit apparatus has one or more non-magnetic conduits extending therethrough. The conduits, in turn, each house a conductor. The conduits are accessible from outside the enclosure. By placing a hook-on type ammeter over the conduit, the current in the conductor may be measured. In one embodiment, the conduit apparatus is a component that mounts within a load center or other electrical enclosure. In another embodiment, the conduit apparatus is an integral portion of such an enclosure.

Abstract: A device for sensing an alternating current includes a circuit board on whose surface is arranged a planar conductor path in which a current to be measured flows. Applied on the opposite surface of the conductor path is a planar coil in which the change in the current to be measured induces a voltage. The device is advantageously used in a converter having a direct-current voltage link circuit.

Abstract: To perform broadband current detection. The current detector of the present invention comprises first terminal, which receives power source current; second terminal, which feeds output current to an outside current; third terminal, which outputs pilot current having a predefined relationship with this output current; and fourth terminal, which has reference potential; as well as first component connected between the first and second terminals; vertical connection of first and second baluns and coupled with the first component; grounded capacitive component at the connecting part of the first and second baluns; and grounded capacitive component at the output part of the second balun. The properties of the instrument for measuring impedance and the apparatus for measuring power are improved by using the current detector of the present invention.

Abstract: A load sensing circuit detects an input load signal across a transformer based on changes in electric current conducted through a primary winding of the transformer. A triangle wave signal is provided to the primary winding of the transformer to induce a corresponding signal at a secondary winding of the transformer. The signal at the secondary winding varies as a function of the input load signal to effect corresponding changes in the electric current conducted through the primary winding. An output circuit provides an indication of the value of the load signal based on the current conducted through the primary winding.

Abstract: A current determiner having an output at which representations of input currents are provided having an input conductor for the input current and a current sensor supported on a substrate electrically isolated from one another but with the sensor positioned in the magnetic fields arising about the input conductor due to any input currents. The sensor extends along the substrate in a direction primarily perpendicular to the extent of the input conductor and is formed of at least a pair of thin-film ferromagnetic layers separated by a non-magnetic conductive layer. The sensor can be electrically connected to electronic circuitry formed in the substrate including a nonlinearity adaptation circuit to provide representations of the input currents of increased accuracy despite nonlinearities in the current sensor, and can include further current sensors in bridge circuits.

Abstract: A measuring configuration for transforming an electric variable on a high-voltage line into a measurable variable. An electrically conductive hollow body that can be connected to a ground potential is connected to a hollow insulating body so as to produce in the interior a cavity through which an electric conductor is guided. In this case, a section of the insulating body at an end of the hollow body side is walled around externally with an electrode body. An electric transformer externally surrounds the hollow body. The measuring configuration is explosion-proof and can be produced in a comparatively cost-effective fashion.

Abstract: An arrangement for measuring alternating or direct current based on the magnetic field generated by the flow of the current includes a sensor for sensing the magnetic field generated by the flow of the current, a measuring circuit for determining the current from the magnetic field, and a device for sensing an external parasitic magnetic field and producing a correction signal applied to the measuring circuit for accurate measurement of the current.

Abstract: A novel toroidal core current sensor for an AFCI/GFCI Circuit Breaker and a test circuit configuration based thereon is disclosed. The current sensor has a plurality of load current conductors extending therethrough. One of the conductors carrying a load current has a portion of the load current shunted outside the core through a shunt conductor which is connected in parallel with one of the load current conductors. The secondary of the transformer outputs a current value proportional to and substantially less than the load current which may be, for example, detected by a control circuit which compares the current value against predetermined stored criteria and operates a circuit breaker accordingly. In the test circuit a power source and a waveform generator are provided for operating an AFCI\GFCI circuit breaker having the shunted current sensor and injecting test signals into a terminal of the circuit breaker.

Abstract: In a method of measuring a current, the current is received at a winding and permitted to flow through the winding, which is wound about a core. The flow of the current through the winding induces a first magnetic flux that flows through the core and that tends to drive the core into saturation. The core is actively reset to prevent the core from being saturated at least during selected time intervals. The current is then measured during at least a plurality of the selected time intervals. In a particularly preferred embodiment, the winding is a primary winding and the resetting step includes applying a reset voltage to a secondary winding using a source that is coupled to the secondary winding. The reset voltage determines a rate at which a net magnetic flux that flows in the core changes in a direction away from saturation. Advantageously, the current sensing method has a tremendous dynamic range.

Abstract: A current sensor assembly includes a sensor coil, an electrostatic shield coil, a core, a housing, and a magnetic shield. The sensing coil, electrostatic shield coil, core, housing, and magnetic shield can be of toroidal symmetry and arranged coaxially about a pair of primary current conductors. The conductors can be either asymmetric or symmetric with respect to the geometric center of the remaining sensor assembly. The core and a secondary winding make up a current sensor. The core is cylindrically shaped and fabricated of non-magnetic material. The secondary winding is wound over the cylindrical core to form a toroidally shaped winding. When assembled into the current sensor assembly, the core and windings are disposed around two single turn primary windings through which AC currents to be measured flow. Alternatively, the conductor can be flat and the sensor can be a solid state sensor that includes an electrostatic shield.

Abstract: A quick connect power line metering and communication apparatus and method for measuring energy consumption or other electrical characteristics on a power line and for communicating the measured information to a remote location. The apparatus includes a housing, and a coupling mechanism attached to the housing to physically connect the housing to a suspended portion of the power line, and to provide an electrical connection to at least one conductor of the power line. A metering apparatus is provided within the housing, and is electrically coupled to the power line conductors via the coupling mechanism to measure energy consumption. The metering apparatus includes a current monitor and a voltage monitor to measure the energy consumption on the power line. A communications apparatus is provided within the housing, and is electrically coupled to the metering apparatus. The communications apparatus communicates information corresponding to the measured energy consumption to a remote location.

Abstract: A signal sensor is provided that includes a substrate, an input port formed on the substrate as a substantially linear conductive element, and a sensing port formed on the substrate adjacent to the input port. The sensing port is influenced by magnetic flux emanating from the input port such that a sense signal is generated in the sensing port. The sensing port can have as little as one sensing loop disposed on one side of the input port, but preferably includes at least one sensing loop on each side of the input port. The sensing loops on each side of the input port have an opposite sense of turn so that the sense signals in each loop are additive. A cross-over connector provides an electrical connection between the loops of the sensing port on opposite sides of the input port. The cross-over connector can be an underpass crossing below the input port or an overpass crossing above the input port. If desired, multiple sensing loops can be formed on each side of the input port.

Abstract: An apparatus for use in sensing electrical current in a conductor includes a flux concentrator and one or more magnetic flux sensors. The flux concentrator includes main sections which are formed of a magnetic material. One or more intermediate sections, formed of a nonmagnetic material, extend between the main sections of the magnetic flux concentrator. The main sections and intermediate section of the magnetic flux concentrator cooperate to at least define an opening through which a portion of the conductor extends.

Abstract: A method and apparatus for detecting motor contamination utilizes zero-sequence currents in a sensing coil to detect contamination. The detector includes a current transformer comprised of a magnetic core and a sensing coil wound about the core. A processor processes a sub-trip level current from the sensing coil to detect contamination. Three contamination detection methods, which may be used separately or in desired combination, are based on current trends, current leakage, and current clustering. An alarm is triggered when predetermined alarm criteria have been met.

Abstract: A remote non-invasive current sensing apparatus for sensing current being carried through a conductor located beneath or on top of a surface. First and second magnetic field sensor elements are placed in operative connection with the surface. There is no requirement to insert the sensor element within the current line by cutting the line or to encircle/enclose the conductor within a coil or other element. The second magnetic field sensor is located a predetermined distance from the first magnetic field sensor. The outputs of the magnetic field sensors are amplified and then provided to a processor which calculates the amplitude of the current in the conductor and the distance of the conductor is from the surface.

Abstract: A magnetic flux concentrator shield is disclosed for use with magnetic flux sensors in an overload relay for multiple pole electromagnetic contactor applications. The magnetic flux concentrating shield has slotted layers of laminated members. The layers have a series of pole shielding slots which concentrate magnetic flux generated by conductors carrying current through conductor apertures in each layer. Magnetic flux sensors are positioned within air gaps within each pole of the shield. By shielding the generated magnetic flux from reaching the magnetic flux sensor of another pole, the pole shielding slots minimize cross pole magnetic flux sensor interference.

Abstract: A bidirectional current sensor including a transformer having a primary winding arranged in a circuit likely to be run through by a current to be measured, a secondary winding, an impedance for demagnetizing the transformer, and a measuring resistor connected to the secondary winding via a switch controlled to be on when a current is likely to run through the primary winding. It applies in particular to the detection of currents in bidirectional voltage converters.

Abstract: A device for non-contact, non-invasive measurement of current or power in a wire, cable or conductor includes a small coil having multiple turns and no ferromagnetic core. The coil may be secured to a wand or housing adapted to be used to place the coil in close proximity to the wire, cable or conductor, whereby a voltage is induced in the coil. An amplifier may be utilized to increase sensitivity. A readout indicates the magnitude of the induced voltage, and a scaling device renders the readout display indicative of the current or power in the wire, cable, or conductor. To measure direct current flow, the coil may be vibrated or rotated adjacent to the conductor to cause the magnetic field to induce a voltage in the coil. The readout may comprise a digital display, a series of light emitting devices, or a flashing light emitting device having a flash rate proportional to the magnitude of the voltage. The device may be constructed in a wand or pen-like fashion, with the coil incorporated into the wand.

Abstract: A connectionless signal detection device for conductive cables contains one or more bifurcated, wire-wrapped magnetic core assemblies positioned axially in an elongated alignment channel, allowing the device to accurately detect the transmission of signals or information on conductive cables using an inductive method. Each assembly section of the magnetic core assembly is wrapped with a conducting wire, with opposed assembly sections electrically connected in a bucking configuration, that is, the output side of one assembly section is connected to the output side of the opposing assembly section. The assembly sections either mate directly at their intersections or alternately operate with a dual gap. The elongated alignment channel opens and closes to allow positioning of the magnetic core assemblies concentrically about a conductive cable.

Abstract: A device (16) for sensing current flowing in a generally flat plate structure (10) contains a magnetic head (18) and signal processing circuitry (20). The magnetic head (a) senses changes in current-induced magnetic flux as the head is positioned over the plate structure and (b) provides a head output signal. The signal processing circuitry processes the head output signal to produce a data signal indicative of how much current appears to flow in the plate structure below the head. A driving voltage, which typically varies in a periodic manner to produce a characteristic signature, is applied to a primary conductor in the plate structure. A location sensor, typically formed with a light source (100) and a light sensor (102), detects the position of the magnetic head relative to the plate structure. A gas-cushion mechanism (80-98) controls the height of the head above the plate structure.

Abstract: Voltmeter for medium/high-voltage devices, which has a surface-wave device (21) as the voltmeter element proper. This surface-wave device is incorporated into an impedance voltage divider (5, 6), of a type known in principle, in such a way that both electrical high-voltage insulation is ensured and power supply problems of the voltmeter element which is at high potential are ruled out.

Abstract: An alternating current sensing apparatus includes at least two coils, each having a plurality of turns of wire around a core which defines a linear axis. The coils are mounted in spaced relation on a support surface so as to define an intermediate space, and the coils are electrically coupled additively in series. The coils produce a detectable signal in response to an alternating electrical current passing through a conductor which extends through the intermediate space.

Abstract: A method and apparatus for processing in output of an inductive sensor, including integrating the output of the inductive sensor with an integrator, having a transfer function with at least two zeros, and at least three poles having characteristic frequencies above the zeros, having a greater number of said poles than said zeros, wherein an integration is performed at frequencies above the characteristic frequencies of the poles, and low frequency noise is substantially rejected at frequencies below the poles. The inductive sensor may be, for example, a Rogowski coil.