Change In Induced Voltage Research Articles

Feasibility of tracking involuntary head movement for MRI using a coil as a magnetic dipole in a time-varying gradient

Accurate tracking involuntary head movements is fairly a challenging problem in MR imaging of the brain. Though there are few techniques available to monitor the head movement of the subject for a prospective motion correction, it is still an unsolved problem in MRI. In this theoretical study, we aim to describe an analytical investigation to track head movement inside an MR scanner by calculating the change in induced voltage in the head-mounted coils during the execution of time-varying gradients. We derive an expression to calculate the change in induced voltage in a coil placed in a time-varying gradient. We also derive a general equation to investigate the changes in the induced voltage in a set of coils mounted onto the head for the planar position and orientation of the coils. Each coil is considered as a magnetic dipole with location and sensitivity vectors. The changes of the vectors can track the head movement in the MR scanner by measuring the changes in the induced voltage in the coils. The dipole concept is valid for a wide range of coils. The changes in induced voltage in the coils are linear due to small changes in pose of the head. Movement parameters are estimated from the induced voltage changes. If the random noise voltage is less than 100 μV, it does not significantly affect movement parameters because the change in induced voltage in the coils dominates over the small noise voltage. This method and array of the coils may provide a real-life solution to the long-standing problem of head motion during MRI.

Superminiature Eddy-Current Transducers for Studying Steel to Dielectric Junctions

We applied eddy current transducer (ECT) to examine the electric steel parameters. The research topic is considered relevant due to the need for evaluation and forecasting of safe operating life of electric steel products. A scheme that uses a computer as a generator and receiver of signals from windings is proposed. Having the designed transducer as a basis, a hardware-software complex was built to perform experimental studies of steel at the interface boundary. The article provides the key technical information about the eddy current transformer used and describes the methodology of measurements that makes it possible to control steel to dielectric transition. Test results are reported for a specimen with continuous and discrete measurements taken at different frequencies. The curve of introduced voltage amplitude as a function of sensor location was constructed to study in details the introduced voltage drop at the ferromagnetic-dielectric interface. Presentation of induced voltage changes in the form of mathematical dependences enables to elicit the ways of interaction between electromagnetic field of eddy-current converter and residual magnetic field of steel.

Chiral charge pumping in graphene deposited on a magnetic insulator

We demonstrate that a sizable chiral charge pumping can be achieved at room temperature in graphene/Yttrium Iron Garnet (YIG) bilayer systems. The effect, which cannot be attributed to the ordinary spin pumping, reveals itself in the creation of a dc electric field/voltage in graphene as a response to the dynamic magnetic excitations (spin waves) in an adjacent out-of-plane magnetized YIG film. We show that the induced voltage changes its sign when the orientation of the static magnetization is reversed, clearly indicating the broken spatial inversion symmetry in the studied system. The strength of effect shows a non-monotonous dependence on the spin-wave frequency, in agreement with the proposed theoretical model.

Dielectric and impedance spectroscopy of (Bi0.5Li0.5)(Fe0.5Nb0.5)O3 multiferroics

The polycrystalline sample of (Bi0.5Li0.5)(Fe0.5Nb0.5)O3 was prepared by a solid-state reaction method. Preliminary X-ray structural analysis of the sample suggests the formation of a tetragonal phase with a new unit cell configuration. Dielectric, electrical, impedance and modulus properties of the material were investigated in a wide range of temperature (25–500 °C) and frequency (1 kHz–1 MHz). Two dielectric anomalies observed at 295 °C and 400 °C clearly suggest the existence of magnetic phase transition and two relaxation processes in the system. Dielectric properties have greatly been improved on addition of LiNbO3 to BiFeO3. The appearance of a hysteresis loop at room temperature confirms the ferroelectric properties of the material. The nature of the Nyquist plot confirms the presence of both bulk and grain boundary effects in the material. The ac conductivity was found to obey Jonscher's power law. The dc conductivity variation with temperature follows the Arrhenius equation. The induced voltage changes with the applied magnetic field, showing that the sample is multiferroic.

Resonance line shape, strain and electric potential distributions of composite magnetoelectric sensors

Multiferroic composite magnetoelectric (ME) sensors are based on the elastic coupling of a magnetostrictive phase and a piezoelectric phase. A deformation of the magnetostrictive phase causes strain in the piezoelectric phase and thus an induced voltage. Such sensors may be applied both for static as well as for dynamic magnetic field measurements. Particularly high sensitivities are achieved for operation at a mechanical resonance. Here, the resonance line shape of layered (2-2 composite) cantilever ME sensors at the first bending-mode resonance is investigated theoretically. Finite element method (FEM) simulations using a linear material model reveal an asymmetric resonance profile and a zero-response frequency for the ME coefficient. Frequency-dependent strain and electric potential distributions inside the magnetoelectric composite are studied for the case of a magnetostrictive-piezoelectric bilayer. It is demonstrated that a positive or a negative voltage may be induced across the piezoelectric layer depending on the position of the neutral plane. The frequency-dependent induced electric potential is investigated for structured cantilevers that exhibit magnetostriction only at specific positions. For static operation an induced voltage is obtained locally at positions with magnetostriction. In addition to this direct effect a resonance-assisted effect is observed for dynamic operation. Magnetostriction in a limited area of the cantilever causes a global vibration of the cantilever. Thus, deformation of the piezoelectric layer and an induced electric potential also occur in areas of the cantilever without magnetostriction. The direct and the resonance-assisted pathway may induce voltages of equal or of opposite sign. The net induced voltage results from the superposition of the two effects. As the resonance-assisted induced voltage changes sign upon passing the resonance frequency, while the direct component is constant, an asymmetric line shape and a zero-response frequency result for the ME coefficient. The zero-response oscillator frequency may be below or above the resonance frequency. The calculated FEM resonance line shapes are fitted successfully to a superposition function of a constant component and a resonant component with a Lorentzian line shape. Equivalence of the superposition function line shape to a Fano resonance profile is derived for frequencies around the resonance. Fano resonances are ubiquitous in physics occurring due to the constructive and destructive quantum interference of two different scattering pathways, e.g., for photons or electrons. The superposition fit parameters describing the resonance line shape are calculated as a function of the cantilever substrate thickness. The inclusion of loss by adjustment of the damping parameter is discussed. The results derived here also are applicable to higher order modes or longitudinal resonance modes.

Selective calcium ion detection with functionalized ZnO nanorods-extended gate MOSFET

Zinc oxide nanorod-extended gate field effect transistor (MOSFET) is demonstrated for the detection of calcium (Ca 2+) ions. ZnO nanorods were grown on the surface of a silver wire to produce an electrochemical nanosensor for selectively detecting Ca 2+. The electrochemical response from the interaction between the ZnO nanorods and Ca 2+ in an aqueous solution is coupled directly to the gate of a field effect transistor (MOSFET). The induced voltage change on the gate results in a measureable current response. In order to adapt the sensors for Ca 2+ ions measurements in biological fluids with sufficient selectivity and stability, a plastic membrane coating containing ionophores was applied on the nanorods. The sensor exhibited a linear response within the range of interest from 1 μM to 1 mM. This work demonstrates a simple technique for sensitive detection of Ca 2+ ions by efficient transfer of the chemical response directly to a standard electronic component producing a low impedance signal.

Ca 2+-dependent and Ca 2+ release-dependent excitation in leech photoreceptors: evidence from a novel “ inside-out” cell model

We have developed a novel, electrophysiologically intact and light-sensitive “ inside-out” cell model (IOCM) of microvillar photoreceptors of the leech Hirudo medicinalis. Light responses recorded from the IOCM with sharp microelectrodes are depolarizations with amplitudes of up to 50–60 mV. In darkness, graded elevations of the free Ca 2+ concentration in the “intracellular medium” (ICM) reversibly increase the conductance of the microvillar membrane leading to Ca 2+-induced graded voltage changes up to ∼50 mV. The threshold for Ca 2+-induced voltage changes is ∼0.06 μM, EC 50 is ∼1.2 μM, and saturation occurs at ∼20 μM free Ca 2+. Small Ca 2+ elevations (<0.6 μM) produce discrete waves of depolarization resembling quantum bumps. Stimulating IOCMs with short (20-ms) and long (5-s) light stimuli produces transient light responses (repolarization within ca. 200 ms) in an ICM containing only 10 nM free Ca 2+. At 0.44 μM free Ca 2+ in the ICM, the microvillar membrane depolarizes by 10–20 mV and responses to 5-s light steps have an initial transient component and a plateau component, similar to responses in intact cells. Generation of the plateau component in IOCMs is suppressed by heparin and cyclopiazonic acid (CPA), agents that block inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3)-induced Ca 2+ release from and Ca 2+ uptake into the endoplasmic reticulum (ER). These results indicate that there is a Ca 2+-dependent conductance in the microvillar membrane and that the light-induced Ins(1,4,5)P 3- and Ca 2+ release-mediated intracellular Ca 2+ elevation in leech photoreceptors contributes to the generation of the receptor potential, particularly the plateau component of responses to long steps of light.

Reflection Induced Voltage Change of Surface Emitting Laser for Optical Probing

We demonstrate a novel optical probing technique using the reflection-induced change in voltage of a GaAs vertical cavity surface emitting laser (VCSEL). We present the modeling and experiment of the VCSEL based probing. A two-dimensional image probing is successfully demonstrated.

Theoretical analysis of the amplification of synaptic potentials by small clusters of persistent sodium channels in dendrites

We extend on the work developed by R.R. Poznanski and J. Bell from a linearized somatic persistent sodium current source to a non-linear representation of the dendritic Na +P current source associated with a small number of persistent sodium channels. The main objective is to investigate the modulation in the amplification of excitatory postsynaptic potentials (EPSPs) in dendrites studded with persistent sodium channels. The relation between membrane potential ( V) and persistent sodium current density ( I NaP) is approximated heuristically with a sigmoidal function and the resultant cable equation is solved analytically using a regular perturbation expansion and Green’s function techniques. The transient simulated (non-evoked) response is found as a result of current injection in the form of synaptically induced voltage change located at a distance from the recording site in a cable with a uniform distribution of ion channel densities per unit length of cable (the so-called `hot-spots') and with the conductance of each hot-spot (i.e., number of channels per hot-spot) assumed to be a constant. The results show an amplification in the observed EPSPs to be compatible with the experimentally derived estimates, and in addition a saturation in the amplification is observed indicating an optimum number of ionic channels.

Degradation mechanisms in organic light emitting diodes

Lifetime measurement results on organic light emitting diodes (OLEDs) are presented and analyzed. The drive voltage tends to increase and the luminance tends to decrease with operating time. Upon the reversal of the polarity of the external field, the voltage trends are reversed almost completely and the luminance trends undergo weak changes. It is suggested that mobile ions can be the cause of the observed voltage changes. Mobile ion transient equations are solved and their time-dependent distributions are obtained. The mobile ion induced voltage changes are also calculated and fitted to the experiment. The agreement between the calculation and experiment suggest that the mobile ions can indeed be the origin of the observed device degradation phenomenon.

Amplification and linearization of distal synaptic input to cortical pyramidal cells.

1. Computer simulations were used to study the effect of voltage-dependent calcium and potassium conductances in the apical dendritic tree of a pyramidal cell on the synaptic efficacy of apical synaptic input. The apical tuft in layers 1 and 2 is the target of feedback projections from other cortical areas. 2. The current, Isoma, flowing into the soma in response to synaptic input was used to assess synaptic efficacy. This measure takes full account of all the relevant nonlinearities in the dendrities and can be used during spiking activity. Isoma emphasizes current flowing in response to synaptic input rather than synaptically induced voltage change. This measure also permits explicit characterization of the input-output relationship of the entire neuron by computing the relationship between presynaptic input and postsynaptic output frequency. 3. Simulations were based on two models. The first was a biophysically detailed 400-compartment model of a morphologically characterized layer 5 pyramidal cell from striate cortex of an adult cat. In this model eight voltage-dependent conductances were incorporated into the somatic membrane to provide the observed firing behavior of a regular spiking cell. The second model was a highly simplified three-compartment equivalent electrical circuit. 4. If the dendritic tree is entirely passive, excitatory synaptic input of the non-N-methyl-D-aspartate (non-NMDA) type to layers 1, 2, and 3 saturate at very moderate input rates, because of the high input impedance of the apical tuft. Layers 1 and 2 together can deliver only 0.25 nA current to the soma. This modest effect is surprising in view of the important afferents that synapse on the apical tuft and is inconsistent with experimental data indicating a more powerful effect. 5. We introduced in a controlled manner a voltage-dependent potassium conductance in the apical tuft, gK, to prevent saturation of the synaptic response. This conductance was designed to linearize the relationship between presynaptic input frequency and the somatic current. We also introduced a voltage-dependent calcium conductance along the apical trunk, gCa, to amplify the apical signal, i.e., the synaptic current reaching the soma. 6. To arrive at a specific relationship between the presynaptic input rate and the somatic current delivered by the synaptic input, we derived the activation curves of gK and gCa either analytically or numerically. The resultant voltage-dependent behavior of both conductances was similar to experimentally measured activation curves.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage-dependent nonlinearities in the membrane of locust nonspiking local interneurons, and their significance for synaptic integration

Some aspects of the electronic and active membrane properties of nonspiking local interneurons were studied in isolated locust thoracic ganglia, using the switched current- and voltage-clamp techniques in neuropilar recordings. The average transmembrane potential (Vr) of the interneurons was -58 +/- 6mV (n = 85), and the input resistance (in the linear region of the current-voltage curve) was 16.5 +/- 8 M omega (n = 19, range 8 to 32 M omega). The membrane and equalizing time constants were estimated from charging curves evoked by the injection of low density hyperpolarizing current pulses from about -80 mV, i.e., from voltages in the linear region of the I-V curve. The curves yielded 2 time constants (tau m and tau l) whose average values were 33.2 +/- 9 msec and 3.3 +/- 1 msec (n = 18), respectively. The mean specific membrane resistance is therefore about 33 k omega.cm2, assuming that the membrane capacitance is ca. 1 microF.cm-2. An outward rectification was always observed upon depolarization from potentials more negative than Vr and was accompanied by a decrease in input resistance and membrane time constant. The "resting" membrane, for example, had a time constant of 26.4 +/- 8 msec (n = 31). This outward rectification was due to at least 2 conductances with different inactivation kinetics, similar to the transient "A" and "delayed-rectifier" potassium conductances. No inward rectification was ever observed upon injection of hyperpolarizing current. In about 60% of the recordings, an active and TTX-resistant depolarizing process could be evoked by rapid depolarization around Vr. The voltage-dependent properties of the membrane of the nonspiking local interneurons had dramatic effects on the shape and time course of natural or evoked unitary PSPs. The half-width of EPSPs, for example, decreased by a factor of 7.5 if the membrane potential was shifted from -93 to -50 mV. When the membrane potential of an interneuron was altered with a triangular current waveform, the reduction of tonically occurring IPSPs depended more on the sign and rate of the induced voltage change than on the absolute transmembrane potential. For 2 identical instantaneous values of membrane potential, for example, the reduction of the PSPs was greater during the depolarizing phase than during the hyperpolarizing phase of the current waveform. The possible nature of the active membrane conductances underlying the nonlinear electrical behavior of the membrane is discussed, together with their functional significance for local circuit synaptic integration.

Parametric Effects on the Induced Voltages on Overhead Lines by Lightning Strokes to Nearby Ground

Outages of low-voltage overhead lines are caused primarily by lightning strokes to nearby ground. These lightning strokes are generally known as indirect lightning strokes. Because of the complex interaction among the various line and lightning parameters, predicting the worst possible severity of indirect lightning strokes on an overhead line is difficult. The paper discusses the effects of some of these parameters, based on the work previously presented. [1,2] The effects of the following parameters on the induced voltage are discussed: 1. Line parameters a. Height of the line above ground, and b. Distance along the line from the struck point. 2. Lightning-stroke parameters a. Return-stroke current, b. Return-stroke velocity, c. Perpendicular distance of lightning stroke from overhead line, and d. Height of cloud charge center. The magnitude of the induced voltage will be directly proportional to the height of the overhead line above ground. In general, the severity of an indirect lightning stroke is not a strong function of the height of the cloud charge center; however, the waveshape of the induced voltage changes considerably for low cloud heights. The analysis shows that the highest induced voltage on an overhead line by a nearby lightning strike to ground may not appear at the point directly opposite the stroke. This phenomenon has also been observed indirectly during field investigations where distribution transformers were damaged at points on the line away from the location of the lightning stroke to ground.

Electrical Membrane Properties and Circadian Rhythm in Extensor Cells of the Laminar Pulvini of Phaseolus coccineus L.

Circadian changes of three parameters of electrical properties in extensor cells of the laminar pulvini of Phaseolus coccineus L. have been recorded by standard microelectrode techniques. In the low turgor state of the cells, the resting potential and the CN(-) (1 millimolar NaCN) induced depolarizations are large, while the K(+)-induced voltage changes are small, and vice versa in the high turgor state. These results demonstrate that during a circadian cycle the low turgor state of the extensor cells is correlated with the active state of an electrogenic pump. This correlation can be explained by electroosmosis.

Effects of maintained depolarization of presynaptic neurons on inhibitory transmission in lobster neuropil

1. Intracellular microeleotrode recordings were obtained from somata of the pre- and postsynaptic neurons of each of four neuron pairs in the stomatogastric ganglion ofPanulirus argus. The microelectrodes were incorporated into a bridge circuit, permitting simultaneous recording and current passing. 2. The following cell pairs were investigated: I. Pyloric Dilator—Pyloric Neuron (PD-PY); II. Anterior Median—Gastric Neuron (AM—GM); III. Large BPSP—Lateral Cardiac or Posterior Gastro-pyloric (EX—LC/GP; IV. Large EPSP—Gastric Mill (EX—GM). The postsynaptic effects of current-induced variations in presynaptic membrane potential were studied in each neuron pair under a variety of experimental conditions. 2. In two cell pairs, the PD—PY and the AM—GM, action potentials initiated antidromically in the presynaptic element did not evoke postsynaptic potentials. If the postsynaptic neuron was simultaneously depolarized through the soma electrode postsynaptic potentials were observed. It is suggested that in the normal functioning of the ganglion local presynaptic depolarizations as well as spikes contribute to transmitter release. 2. In the EX—GM and the EX—LC/GM cell pairs, presynaptic depolarization that did not initiate action potentials nevertheless evoked postsynaptic inhibition and hyperpolarization for as long as the presynaptic current was maintained. Control experiments showed that presynaptic hyperpolarization had no postsynaptic effect. 2. The hyperpolarizing effect of presynaptic depolarization could be blocked by picrotoxin in parallel with blockade of other IPSPs in the ganglion. 2. The experimentally induced voltage changes in the presynaptic terminals are probably comparable to those resulting from synaptic input to the neuron. These experiments support the hypothesis that, in life, depolarization of presynaptic terminals that are subthreshold for action potentials may facilitate or evoke transmitter release. Such presynaptic, nonspike potentials may play a significant role in the modulation of synaptic transmission in neuropil.