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In our courses the important topic 'transfer impedance' is explained with the help of demonstrations in the class room and in lab assignments. Cables, connectors and equipment are taken as example. The influence of shape, resistance and induction is shown. Measurements in time domain cover the speed range up to several tens of nanoseconds with a conventional square wave generator and oscilloscope. The results are compared to expectations from the theory presented in the course.

We report a 511‐keV photon flux enhancement that was observed inside a thundercloud and is a result of positron annihilation. The observation was made with the In‐flight Lightning Damage Assessment System (ILDAS) on board of an A340 test aircraft. The aircraft was intentionally flying through a thunderstorm at 12‐km altitude over Northern Australia in January 2016. Two gamma ray detectors showed a significant count rate increase synchronously with fast electromagnetic field variations registered by an on‐board antenna. A sequence of 10 gamma ray enhancements was detected, each lasted for about 1 s. Their spectrum mainly consists of 511‐keV photons and their Compton component. The local electric activity during the emission was identified as a series of static discharges of the aircraft. A full‐scale Geant4 model of the aircraft was created to estimate the emission area. Monte Carlo simulation indicated that the positrons annihilated in direct vicinity or in the aircraft body., Key Points A sequence of 10 X‐ray pulses was observed from an aircraft inside thundercloud at 12‐km altitudeThe pulses contain strong enhancement of the positron annihilation line (511 keV)Each pulse lasts for 1 s and is accompanied by a static discharge from the aircraft

In order to retrieve the lightning attachment points, 12 sensors distributed over the aircraft continuously monitor the local electric and magnetic field. The selection of the number and positions of the sensors is described in [4]. The ILDAS is triggered by sudden changes in the field strength in the aircraft. The sensors then determine the local magnetic fields, retaining pre-and post-trigger data over the time span of about 1 s. After landing, the data are compared with possible field distributions derived for a number of current path scenarios [3], and the most likely entry and exit regions are determined.

The In-flight Lightning Damage Assessment System ILDAS has been presented in EMC Europe in 2012. ILDAS can determine the lightning current distribution on an aircraft with high resolution in time and amplitude. Later the system was extended and included two x-ray detectors to measure the high-energy radiation that is generated in thunderstorms and by lightning proper. It has been flown for many times now. The paper presents a few selected data.

Aluminium is widely used as conductor material for power cables, but corrosion taking place under specific circumstances can have impact on its reliability. Aluminium corrosion under the influence of an alternating current was studied experimentally. Submerged cable segments with inflicted damage exposing the conductors resulted in a continuous corrosion of the conductor material. Effects of temperature and applied voltage were studied by observing corrosion of an aluminium wire. These parameters strongly affect the corrosion rate. Comparison with copper showed that aluminium is far more susceptible to corrosion. The impact of corrosion on the reliability of the cables is discussed.

Thin films for material characterization and noise tests are preferentially manufactured with rectangular electrode shapes. In the companying paper Vandamme presents expressions for resistance and geometrical noise factor fQ for the case of circular electrodes. The present paper employs numerical conformal mapping to obtain accurate values for rectangular and circular electrodes. The sample size is either assumed very large, or is taken comparable to the electrodes.

A novel sensor for the detection of the lightning current through the fuselage of an aircraft has been tested on an A320 aircraft. An accurate method-of-moment model of the window edge provided reliable calibration of the sensor for external fields. The data have been analyzed and the good performance of the sensor in sensitivity and bandwidth is demonstrated.

A novel sensor is described for the detection of the lightning current through the fuselage of an aircraft. The sensor relies on the penetration of the magnetic field through fuselage openings and can be embedded in a window inside the aircraft. The sensor combines good sensitivity with sufficient bandwidth to record the lightning transient current. Guidelines for the position are derived from a mathematical analysis for a circular window.

In a recent paper (G. Bargboer and A. P. J. van Deursen, IEEE Trans. Electromagn. Compat., vol. 52, no. 3, pp. 684-90, Aug. 2010) we dealt with current injection measurements to test the lightning protection system of a newly built pharmaceutical plant. In a tentative extrapolation, the measurements were extrapolated to actual lightning. Here, we extend the model and calculate the response of the installation on lightning currents and include resonances in the cable trays and test cables contained in it. It turned out that suspension rods between roof support and cable tray were indispensable to suppress the resonances. The time dependence of the present results and the earlier simplified model given in differ greatly, but the amplitudes of induced voltages appear to be of the same order of magnitude.

A newly built pharmaceutical plant has been investigated by measurements. Currents of 0.3 kA were injected in the lightning protection grid on the roof. Inside the building, test cables of 100 m length followed a path typical for cables belonging to the installation. We measured induced cable currents and voltages. A reduced model of the building incorporated most of the designed current paths. Measurements and model showed that the roof steel skeleton carried about 80% of the current and the intended lightning conductors 20%. The calculated current through a cable support was larger than measured. This is explained by also considering a nearby nonintended conductor. For three types of cables, we determined the transfer impedances. The measurements and model have been combined and extrapolated to actual lightning.

Conductance fluctuations are easily probed by passing a constant current through a pair of driver electrodes and measuring the voltage fluctuations across a pair of sensor electrodes. We calculate the geometrical factor for these fluctuations from the current distribution derived by conformal transformations. In four-electrode configurations on a flat square sample this factor depends only slightly on electrode size, whereas a strong dependence is found for two-electrode configurations. The calculations also give accurate values for the resistance, which allows a comparison with the thermal noise.

A 2 MV lightning surge generator is a powerful source of interference, in particular when the voltage waveform is chopped by a shorting spark gap. We not only do need to protect the sensitive diagnostic electronic equipment that operates on small signals, but we also should guarantee its flawless operation during the discharge. The paper discusses the setup, the EMC measures taken, and the results of verification measurements. Some typical results obtained in the research with this instrument will be shown as well.

Large-scale tests at the new Harbor Railway Line in The Netherlands asked for a dedicated measuring system for all currents at eight cross sections of the railway. The differentiating/integrating measurement scheme with adapted versions of Rogowski coils and special integrators fulfilled the requirements with respect to accuracy, bandwidth, electromagnetic compatibility, life-time, and environmental constraints. Three types of current sensors were developed; their air-core guaranteed linearity over the full current range up to 20 kA. Most importantly, the sensors allowed unimpeded rail transport. The paper discusses the sensors, the rationale, and tests, as well as the integrators. The system has been successfully employed, before and after the large-scale tests, in Luxembourg and Portugal. A few out of much data gathered show the broad applicability of the system.

The effect of a nearby U-shaped metal cabinet panel on the inductive common-mode coupling of a printed circuit board (PCB) is calculated in a two-dimensional (2-D) model by Schwarz-Christoffel (SC) transformations for doubly connected regions. Such an open metal cabinet can significantly reduce the mutual inductance between the microstrip lines on the PCB and the common-mode circuit if the cabinet is properly connected. But it may also increase the coupling for balanced pairs of tracks, in particular where the coupling for a single track varies strongly with position. Details of the SC approach are presented. The SC results agree with those of a static method of moments approach, but also clearly indicate the accuracy limits of the latter method.

Radio base stations are often mounted on towers of power transmission lines. They are usually powered from the low-voltage network through an isolating transformer, to separate the high- and low-voltage networks. The isolating transformer ensures security at customers' premises in the case of nearby power faults and particularly in case of lightning striking, which is a frequent cause of flashover of the high-voltage insulators and a subsequent phase-to-ground fault. To optimize such protection, a better understanding of the current distribution to the ground in the initial phase of the lightning striking is required. This work presents a computer simulation method and its results for the lightning current distribution between the sky wires, grounding systems, and power cable network. What is of special concern is the part of the lightning current that might be led into the metal-shielded distribution cable network. A rigorous electromagnetic modeling approach was applied for the analysis of the grounding effects in a network of coated and uncoated low- and medium-voltage cables. The analysis was then applied to a practical case.

Fully shielded tracks can be embedded in multilayer printed circuit boards by a novel manufacturing technique. The paper presents a numerical analysis of the influence of dimension tolerances on the characteristic impedance. For partly shielded tracks the coupling between tracks and the environment is studied as well. The analysis relies on a two-dimensional approach, using Conformal Transformations and a static Method of Moments.

Low-voltage (LV) applications mounted on high-voltage (HV) towers may pose a safety problem in the low voltage network when a flashover occurs over the high-voltage insulators. To study the effects experimentally, a 150 kV system was made available. On a high voltage tower carrying a cellular phone base station, one phase insulator was shorted to the tower. A current of about 200 A was injected into that phase at a large distance. We measured the current distribution and the relevant voltages in the low voltage net. The experimental current data agree with EMTP calculations. Appropriate measures to guarantee safety for persons and electronic equipment are discussed.

A self-optimizing discretization adjustment scheme is described for boundary element calculations. The aim is to obtain high accuracy with limited computer resources. Examples will be given of two-dimensional static field calculations, where small capacitances are accurately determined in presence of others with three up to five orders larger values. To demonstrate the power of the scheme, the simplest basis function is used: a constant charge density per boundary segment. Several results are compared to exact calculations, and fast convergence and good agreement are demonstrated.

Common-mode (CM) currents on cables attached to printed circuit boards of various complexity have been measured. The boards have a continuous ground plane, with tracks on both sides. The experimental results are compared to calculations with a transmission-line (TL) model. Expressions for the coupling parameters in the TL model have been derived with conformal transformations and the method of moments. Measurements up to 1 GHz agree with the TL model, showing that this model can be used as a tool for an early EMC analysis during the lay-out and routing of a board.

Metallic conduits provide mechanical support for cables, and may also protect the cables against interference caused by currents induced by external electromagnetic fields. The conduit transfer impedance Z/sub t/ then is a good measure for the provided protection. We studied Z/sub t/ by means of conformal transformations and of the method of moments in a quasistatic, two-dimensional (2-D) approach. Practical U-, H-, L-, and T-shaped conduits are discussed. Conformal transformations provide simple engineering expressions for the mutual inductance part of Z/sub t/ for cable positions inside the conduit. The method of moments formulation results in useful approximations for the crossover frequencies in Z/sub t/ between DC and high frequency. Measurements on U-shaped conduits confirm the calculations.

When lightning strikes an aircraft, the current distribution and the attachment points are of interest. Simple window sensors mounted in different windows along the fuselage provide signals over the local magnetic field due to lightning current, its orientation and time behavior.

Current diagnostics of a high voltage corona discharge setup are analyzed, because the presence of high amplitude oscillations prohibited to obtain a realistic current signal. By means of an electromagnetic computation in FEKO, the observed oscillations have been identified. Current distributions indicated setup improvement by adding a 50Ω damping resistor in the circuit. A realistic current signal is now obtained.

The introduction of 25 kV a.c. traction in the Dutch railway system required a careful study of the compatibility with existing 1500 V d.c. traction and with other systems in the vicinity of the railway. Over the last decade, a number of studies have been carried out, using specially developed measurement systems and methods of analysis. The present paper describes a newly developed Poynting sensor that builds on the earlier developed differentiating/integrating measurements system. Measurements on the railway power system with current injection at e.g. 60 and 48 Hz will be discussed as well.

In the EU FP6 project ILDAS a prototype In-flight Lightning Damage Assessment System is developed for passenger aircraft and helicopter. The project aims to localize the attachment and the severity of the strokes during flight to assist maintenance. A database of events will be constructed to substantiate requirements for aircraft lightning immunity. Inductive magnetic field sensors are put at various positions on the aircraft. The sensors are followed by passive/active integrators to deal with the large dynamic range and bandwidth, and by high speed data conversion/storage. The paper focuses on design and tests of three types of sensors.

Armored installation cables are often intended to be buried in the soil. The benefits of such armor for frequencies up to a few MHz are generally known. We determined the transmission parameters of such a cable with a network analyzer, and measured the shielding effectiveness of the armor up to 1 GHz in a reverberation chamber. A solid shield coaxial cable with a small hole acted as calibration.

In the EU FP6 project ILDAS a prototype In-flight Lightning Damage Assessment System is developed for passenger aircraft and helicopter. The project aims to localize the attachment and the severity of the strokes during flight to assist maintenance. A database of events will be constructed to substantiate requirements for aircraft lightning immunity. Inductive magnetic field sensors are put at various positions on the aircraft. The sensors are followed by passive/active integrators to deal with the large dynamic range and bandwidth, and by high speed data conversion/storage. The paper focuses on design and tests of three types of sensors.

The lightning protection of a pharmaceutical plant has been tested by current-injection on the structure. The study aims at two goals: 1) proof of safety by scaling the currents and voltage to lightning realistic values and 2) a comparison between measurements and a Method of Moments calculation. We measured the current distribution near the source and the induced currents and voltages in test cables. The cables and their 100 m long path were representative for the actual installation. By necessity the model is simplified, because of the large number of conductors, intended and unintended, and the uncertainty of their interconnection. The calculated currents are a factor of 3 larger than measured. © 2009 IEEE.

Power systems often use coated and/or uncoated metallic sheathed cables for medium and low voltage distribution. The metallic sheaths of such cables are connecte to the grounding systems and may have significant grounding effects. In case of uncoated metallic sheathed cables they act as extended grounding electrodes while coated cables connect different grounding systems at transformer stations and customer premises. This usually improves the grounding conditions at the source transformer station but potentially dangerous voltages might be transferred to other locations. This paper analyses the lightning current distribution between the local grounding system, stoked by lightning, and connected cable metal sheaths. It is shown that after the first tens of microseconds local grounding system loses its function and cables take away nearly all, lightning current

The lightning protection of various industrial installations has been investigated by injecting current of smaller amplitudes. Large deviations of the intended current distribution and high values of induced voltages have been observed. Local measures can remedy the deficiencies.

X- or gamma-rays have been produced in the developing discharge of a 2 MV lightning surge generator. The generator provides a voltage waveform with 1.2/50 mus rise/fall time. The waveform is truncated by a spark gap: two sharply pointed electrodes placed horizontally at 0.7 m distance with one electrode grounded. Gap breakdown occurs at about 850 kV. High energies has been recorded with BaF2 and LaBr3(Ce) gamma detectors facing the surge. Aluminum windows limit the detection to gamma's with energy above 17 keV. The scintillator- photomultiplier combination has been calibrated by the 662 keV line of a 137Cs source; the pulse output rise/fall time is 11/23 ns. With positive surges applied to the floating spark gap electrode, hard radiation is observed in 100% to 60% of the surges depending on the electrode's distance to the measurement equipments. The radiation occurred in the leading edge, during the streamer development before a conducting channel was formed between the electrodes. The gamma signal consists of several pulses, well resolved in time. Gamma pulses equivalent to 2-3 MeV were detected. From absorption measurements we conclude that these high energy pulses are pile-up of multiple gamma-quanta, each over 100 keV. The pile-up occurs within the rise time of the detector response, 11 ns. The consistent timing with the HV-surge excluded background radiation as a source for these high intensity pulses. For negative discharges three things change: more surges resulted in gamma-pulses, more gamma-pulses per surge, and more gamma energy. Gamma-pulses up to the equivalent of 30 MeV have been recorded. The high equivalent energies are caused by streamers from the measurement cabinet, right in front of the detector. X-ray was also measured from streamer-corona plasma in a wire-plate reactor. No breakdown occurred during the experiment. A high repetition rate 15/25 ns high HV-pulse with 100 ns width was applied to the wire. The voltage pulse's amplitude was 65 kV with an additional DC offset of +20 kV. Energies equivalent to 45 keV have been recorded.

Commercial passenger aircraft are on average struck by lightning once a year. The in-flight lightning strike damage assessment system (ILDAS) project is to develop and validate a prototype of a system capable of in-flight measurement of the current waveform and reconstruction of the path of lightning current. This paper discusses the various magnetic sensors, their rationale and some tests. Inductive sensors require an integrator in the signal conditioning. Much attention has been paid to the passive integrator, which is the first integrator stage preliminarily responsible for EMC.

A cylindrical electrolytic tank between a set of Helmholtz coils provides a classroom demonstration of induced, nonconservative electric fields. The field strength is measured by a sensor consisting of a pair of tiny spheres immersed in the liquid. The sensor signal depends on position, frequency, and orientation. A low resistance copper ring strongly modifies the magnetic field inside and increases the field outside; the electric field changes accordingly. A metallic ring with a small gap can be used to measure the behavior of the electric field in the vicinity of windings around a transformer core. The measured results are consistent with simple models.

Cellular phone base stations are often placed in the poles of power transmission lines. We consider the case when such base stations are powered from the low-voltage network. Of special concern is the current that might be led through the cable metallic shields to other customers' premises in case of lightning stroke, which is a frequent cause of flashover of the high-voltage insulators and a subsequent phase to ground fault. This paper presents an analysis in the frequency domain of the lightning current distribution between the earth wires, grounding systems and power cable network, in case of lightning stroke at the tower. The models and the analysis are applied to a practical case

This paper describes the consequences of a fault in a medium voltage network on the grounding systems at the LV-side. To study the current distribution and to verify the models, we deliberately introduced one phase to ground faults in the 10 kV floating MV network. The selected site was the end of a 2 km long buried MV cable feeding a small group of houses; the soil there had a high resistivity. The fault current was 127 A. The current distribution and the relevant voltages were measured at several positions in MV and LV net. We compare the results for a TT and a quasi-TN earthing system in the houses, also taking the telecom cable into account. The distribution of the fault current over the MV cable, measured at another MV station at 2.2 km distance, are in agreement with calculations, based on a model which includes the contact of the lead shield of the MV cable with the ground. Current and voltage data are compared with calculations. Eindhoven University of Technology and the Dutch power distribution company NUON, carry out a joint research project on global earthing. In this project we treat different faults and various interference sources e.g. lightning and switching events together.

Conduits of various shapes offer good electromagnetic protection to cables placed inside. As measure of protection, the transfer impedance has been studied; in the frequency range of interest the current adjusts between the low and high frequency distribution in the conduit. Quasistatic two-dimensional calculations are based on the method of moments and on conformal transformations. Practical U, H, L and T shapes are studied mostly. Engineering equations directly give the dominant parts of the transfer impedance for various cable positions inside non-magnetic conduits. Measurements on U conduits confirmed the models.

Since 1991, NUON has applied a TN system for their LV network, low voltage clients are then no longer required to have a separate ground electrode. To guarantee safety for LV clients not only faults in the LV network, but also ground faults in HV or MV grids should be considered, since they may cause large touch voltages in the LV network. A particularly interesting example is a LV system in a HV tower, as is the case with a mobile phone base station. A safety problem may occur in the low voltage network after a flashover of the HV insulators. A fraction of the 50 Hz current will flow into the LV grid. We studied the problem both with an experiment and simulations. In this paper we give a detailed description of our measurements. A new guideline to protect persons and electronic equipment is discussed.

Low voltage applications mounted on high voltage towers may pose a safety problem in the low voltage network when a flashover occurs over the high-voltage insulators. To study the effects experimentally, a 150 kV system was made available. On a high voltage tower carrying a cellular phone base station, one phase insulator was shorted to the tower. A current of about 200 A was injected into that phase at a large distance. The authors measured the current distribution and the relevant voltages in the low voltage network. A experimental data agree with EMTP calculations. Appropriate measures to guarantee safety for persons and electronic equipment are discussed.

The classical lightning conductor, which must prevent fire, has to have a sufficiently small resistance. An analogous condition can be formulated for the new challenge: the protection of sensitive electronics against lightning. In this case, the so-called transfer impedance, which gives the interference voltages across a sensitive input per ampere lightning current, must be made small. The arguments for this approach are described. A theoretical description is available and practical experience has been built up over the years, also in high-voltage (HV) research and in power engineering measurements in the field. This approach was used to greatly improve the lightning protection of several installations: a marine radio station, the peripheral equipment of a nuclear power plant, and an electronic siren for a nationwide public warning system. Detailed investigations were requested by our contract partners, together with suggestions for economically acceptable improvements that could be carried out in reasonable time. Later, the correctness of the renovations had to be demonstrated. In the validation measurements, we injected currents into the installation or into relevant parts thereof. The current waveform was chosen fast enough so that inductive effects determined the current distribution. At present, lightning position and tracking system (LPATS) data on lightning strikes near the marine radio station and the nuclear power plant are available to verify the effectiveness of the protection. After our improvements no more damage was reported.

An inexpensive, simple, and sensitive workbench setup has been developed to determine the transfer impedance Z/sub t/ of shielded connectors. The injection current is measured by an inductive sensor integrated in the setup. The induced voltage is determined by a spectrum/network analyzer. The overall sensitivity is 3 /spl mu//spl Omega/ or better; the frequency range is up to 1 GHz. No special calibration samples are needed. As a demonstration the Z/sub t/ is measured for both a coaxial tube with a small hole in the shield which provides a mutual inductance of 0.1 pH. Improvements in the design of the shield of a telecommunication connector are presented. Results for some common connectors are given; the transfer impedances for these connectors ranges from 1 m/spl Omega/ to more than 100 m/spl Omega/ at 1 GHz.

The transfer impedance Z/sub t/ of a cable is often assumed to be a characteristic of the shield only. We investigate the limits of this assumption by calculations and measurements. The first test cable was formed by a solid copper tube (ldquoshieldrdquo) and a wire inside; several positions, also non-central, were chosen for the wire. The common mode current (between 10 Hz and 100 kHz) through the tube had as return path a single wire, which was placed at several positions, near and at some distance from the shield. A second cable with a braided shield was tested as well. The results show that both the differential mode and the common mode circuit have to be carefully defined for a particular Z/sub t/. Varying either circuit may alter Z/sub t/ drastically. Consequently, the Z/sub t/ of a particular cable measured in a triaxial setup, is a characteristic of that setup and cannot always be used in another setup as in for instance cable bundles

We investigate structure and evolution of long positive spark breakdown; and we study at which stage pulses of hard X-rays are emitted. Positive high-voltage pulses of standardized lightning impulse wave form of about 1 MV were applied to about 1 meter of ambient air. The discharge evolution was imaged with a resolution of tens of nanoseconds with an intensified CCD camera. LaBr3(Ce+) scintillation detectors recorded the X-rays emitted during the process. The voltage and the currents on both electrodes were measured synchronously. All measurements indicate that first a large and dense corona of positive streamers emerges from the high voltage electrode. When they approach the grounded electrode, negative counter-streamers emerge there, and the emission of hard X-rays coincides with the connection of the positive streamers with the negative counter-streamers., 19 pages, 11 figures in Journal of Physics D: Applied Physics 2012

Metallic trays and conduits protect bundles of cables in electrotechnical installations by diversion of interference currents. The relevant parameter is the conduit transfer impedance Z/sub i/. Conformal transformations yield the inductive part M of Z/sub i/ for conduits with noncontacting covers. Analytical approximations for M agree well with measurements.

X-ray emission has been detected occasionally during the streamer-corona propagation in a wire-plate corona reactor open to ambient air. A 65 kV pulse with 15 ns rise time is applied to the wire anode superimposed on a 20 kV dc bias. The duration of the driving voltage pulse (110 ns) is less than 2.5 times the primary streamer transit time. Under this condition no arc discharge occurs between the wire and the cathode plates separated by 6 cm air. The onset of x-ray emission coincides with the initiation of the primary streamers near the wire anode. No x-rays were detected later, during or after the primary or secondary streamer development. X-ray energies ranged between 10 and 42 keV, as detected by a LaBr3 (Ce) scintillator–photomultiplier combination. Time resolved imaging of the streamer propagation highlights the different stages in the streamer discharge process. The energetic electrons originate near the anode, at the moment of streamer initialization.

A lightning surge generator generates a high voltage surge with 1.2 microsec. rise time. The generator fed a spark gap of two pointed electrodes at 0.7 to 1.2 m distances. Gap breakdown occurred between 0.1 and 3 microsec. after the maximum generator voltage of approximately 850 kV. Various scintillator detectors with different response time recorded bursts of hard radiation in nearly all surges. The bursts were detected over the time span between approximately half of the maximum surge voltage and full gap breakdown. The consistent timing of the bursts with the high-voltage surge excluded background radiation as source for the high intensity pulses. In spite of the symmetry of the gap, negative surges produced more intense radiation than positive. This has been attributed to additional positive discharges from the measurement cabinet which occurred for negative surges. Some hard radiation signals were equivalent to several MeV. Pile-up occurs of lesser energy X-ray quanta, but still with a large fraction of these with an energy of the order of 100 keV. The bursts occurred within the 4 nanosec. time resolution of the fastest detector. The relation between the energy of the X-ray quanta and the signal from the scintillation detector is quite complicated, as shown by the measurements., 7 pages, 7 figures

In the ideal case, noise measurements with four contacts minimize the contribution of the contact interface. There is a need to characterize conductance noise and noise correction factors for the different geometries provided with four contacts, as already is the case for resistivity measurements with van der Pauw structures. Here, we calculate the noise correction factors for two geometries with a pair of sensors and a pair of current driver electrodes placed in a square array. The first geometry investigated is a very large film compared to the distance L between four circular electrodes, which are placed in a square array far away from the borders of the film. The second is a square-shaped conductive film with side length L and provided with four quarter-circle corner contacts with radius l. The effect of the conductance noise in the film can be observed between current free sensors in a four-point measurement or between current carrying drivers in a two-point measurement. Our analytical expressions are based on approximations to solve the integrals (J·)2dA and |J|4dA for the voltage noise measured across a pair of sensors, SVQ, and across the drivers, SVD, respectively. The first and second integrands represent the squared dot product of the current density and adjoint current density and the modulus of the current density to the fourth power, respectively. The current density J in the samples is due to the current I passing through the driver contacts. The calculated expressions are applicable to samples with thickness tl0.1L. Hence, the disturbances in the neighborhood of the sensors on J and of the drivers on are ignored. Noise correction factors for two- and four-point measurements are calculated for sensors on an equipotential (transversal noise) with the driver contacts on the diagonal of a square and for sensors next to each other on one side of the square with the drivers next to each other on the other side of the square (longitudinal noise). In all cases the noise between the sensors is smaller and less sensitive to the contact size 2l/L than the noise between the drivers. The ratio SVQ/SVD becomes smaller with smaller contact radius l. Smaller sensors give a better suppression of interface noise at the contacts. But overly low 2l/L values result in overly high resistance between the sensors and too strong a contribution of thermal noise at the sensors. Therefore, equations are derived to calculate the current level needed to observe 1/f conductance fluctuations on top of the thermal noise. The results from the calculated analytical expressions show good agreement with experimental results obtained from the noise in carbon sheet resistance and numerical results. Transversal noise measurements on a square sample with corner contacts are recommended to characterize the 1/f noise of the layer. This is due to the increased current densities in the sample compared to the open structure, which result in easier detection of the 1/f on top of the thermal noise. ©2007 American Institute of Physics

The Fermi surface has been compared for the three compounds LnB6 (Ln  La, Ce and Pr) by measurement of the de Haas-van Alphen effect. The major part is observed to be similar in each compound, most likely consisting of X-centred ellipsoids with a total effective volume of about 1 electron per formula unit. Differences occur in the details of the shape of the FS, clearly seen for PrB6. Large effective masses, m ∗ , are observed for CeB6 and PrB6, m ∗ = 6 and about 1.8, respectively. These have to be compared with m ∗ = 6.0 for LaB6.

Magnetization and magnetoresistivity measurements near 4.2 K on UAl/sub 2/ in fields up to 35 and 25 T, respectively, show a suppression of spin fluctuations in this material between 15 and 20 T. The experiments strongly suggest that the field and temperature effects on the paramagnon contributions are closely related. The validity of various theoretical models on the suppression of paramagnon effects are also discussed in light of these measurements.

Pulsed field de Haas-van Alphen measurements of the Gamma centred part of the Fermi surface in ZrZn2 are presented for fields up to 35T. The spin-split Fermi surface was found to be field dependent. It is argued that the observed de Haas-van Alphen frequency is different from the frequency defined by the Onsager relation. An expression is derived based on the Stoner-Wohlfarth model which describes the difference in spin-up and spin-down frequencies as a function of field strength. The observed difference in the effective mass for the two spin directions can also be explained by this model and it compares well with band structure calculations. An anomalous behaviour of the second harmonic of the majority surface de Haas-van Alphen frequency was observed. A search for other frequencies was carried out but in spite of the high sensitivity, in terms of m*TD, no other orbits were seen.

A method for growing ZrZn2 crystals from the melt is described. In a 1 cm3 sample, which contains large crystals, we measured very high resistivity ratios ϱ 293 K ϱ 4.2 K of 120–131. The lattice constant was 0.73969(5)nm and the zinc-zirconium ratio 1.99. The temperature dependence of the resistivity was also measured. Furthermore, the crystals were characterized by Curie temperature and magnetic moment measurements.

High-field magnetoresistivity (MR) measurements are presented for high-purity samples of TiBe/sub 2/ and UAl/sub 2/. The MR of TiBe/sub 2/ gives strong evidence for scattering due to spin fluctuations. The peculiar features in the MR around 5 T are apparently connected to the maxima in the susceptibility and may provide a clue to a theoretical description of the contributions of spin fluctuations to the resistivity. A value for the spin-fluctuation temperature is obtained and it is in agreement with earlier estimates. The MR of UAl/sub 2/ is consistent with the behavior of a compensated metal. From the high-field behavior of the MR we expect to observe de Haas--van Alphen oscillations above approximately 10 T, but up to 40 T no signals are found. A small negative MR at higher temperatures may be due to spin-fluctuation scattering, but the evidence is not as clear as for TiBe/sub 2/.