Ohmic Contribution Research Articles

Effect of the heart-lung boundary on the magnetocardiogram

Ionic currents flowing during the activation of the heart generate both electric potentials which can be recorded at the body surface as the electrocardiogram or ECG and magnetic fields which can also be recorded at the body surface as what is termed the magnetocardiogram (MCG). In order to apply the MCG to basic research or clinical diagnosis, one must understand the geometric sensitivity of the recorded signals to the cardiac source currents and evaluate the contribution of secondary ohmic body currents relative to the cardiac currents. The present study aims to evaluate the magnitude of the ohmic current contribution by electrically insulating the heart from the body. It is found that volume currents contribute a large part of the MCG signal recorded at distances greater than 10 cm from the heart, but that close to the heart at least one component of the MCG reflects primary cardiac source currents free of interference from volume currents flowing in the body.

Temperature dependence of AuPd thin film alloy contact formation on oxidized silicon substrates

The formation of metallic contacts of very thin vapour-deposited AuPd alloy films on Si(111) substrates covered with thin oxide layers was studied by means of surface analysis techniques (Auger electron spectroscopy, reflection high energy electron diffraction and transmission electron microscopy) as well as by electrical measurements (current density versus voltage characteristics). On deposition of the metals at temperatures below 100°C onto samples covered with native oxide, metal/ insulator/semiconductor-like behaviour was found. An increasing contribution of Schottky-like contacts was observed for deposition temperatures above 100°C, whereas for deposition temperatures above 200°C an increase in the ohmic contribution and a decrease in the photoresponse were observed. These effects were explained by the selective reaction of palladium with the oxide interface and the formation of palladium silicide. Segregation of alloy films produced an intimate contact between the metal overlayer and the silicide. The minimum amount of palladium necessary to produce nearly ideal ohmic contacts was estimated to be a few nanometres. Hardly any ohmic contribution was obtained when the depositions were carried out onto thermally oxidized samples with an oxide thickness of 7 nm, even for substrate temperatures of 620°C.

The non-linear response of NbSe 3 to pulsed electric fields

The non-linear response of NbSe 3 to pulsed electric fields has been measured using a bridge circuit which subtracts the ohmic contribution to the voltage from the total signal. The current-induced metastable states of the charge-density wave (CDW) reported by Gill are confirmed. The periodic voltage induced by dc electric fields is clearly resolved and in some cases may be synchronized with the applied pulses. The periodic voltage comprises a large fraction of the non-linear response, growing to 100% near threshold.

Analysis of Time‐Variation Effects in Water Electrolyzers

An analysis of unipolar electrolyzer equipment is reported which allows estimation of the different contributions to cell voltage of the reversible potential, of electronic and ionic ohmic resistance, and of the electrode overvoltages. A technique is described which allows performance data for a commercial electrolyzer to be decomposed to give the ohmic resistance contribution as well as the sum of the Tafel slopes for the anodic and cathodic processes. Substantial time variation of both the anode and cathode overvoltages was observed, with the rate and magnitude of this variation being dependent on current density. In light of these results, appropriate methods for long‐term evaluation of new electrode materials are discussed, and a reliable long‐term‐test cell is described. Representative data obtained on prolonged operation of cathodic and anodic electrode samples are reported.

Studies in electrocrystallization: Part I. A computer simulation of the effect of ohmic overpotential on the transient response of a three-dimensional progressive nucleation and growth mechanism

A computer simulation is presented of the effect of ohmic overpotential on the transient response of a three-dimensional progressive nucleation and growth mechanism. Distortion of data is described in terms of the shapes of experimental transients, as well as in terms of the characteristic parameters I m and t m. A method is suggested for extrapolating out the ohmic contribution to the overall potential.

A simplified technique for determining the boundary layer voltage loss in MHD generators

MHD generator performance predictions require an accurate determination of the voltage losses in the channel; however, most techniques for determining these losses need substantial calculations and/or computer storage space. This paper proposes a simplified method for calculating the ohmic boundary layer contribution to the overall voltage losses. Voltage drop regions are discussed and a description of the turbulent boundary layer contribution is derived. Appropriate simplifying assumptions on the basic transport and MHD concepts are used to express the conductivity as a function of temperature only. Weighting functions for averaging the resistivity in turbulent boundary layers are determined and the nature of these functions is presented. Our results are compared with more precise descriptions and with experimental results.

Determination of the Ohmic Contribution to the Polarization of Fuel Cell Electrodes

not Available.

Hydrogen Overvoltage on Rhenium and Niobium Electrodes

Polarization curves of the hydrogen evolution reaction are presented for rhenium and niobium electrodes in dilute hydrochloric acid solutions as a function of temperature. The Tafel reaction is the rate‐determining step in the hydrogen evolution reaction on rhenium. The behavior of niobium electrodes is much more complicated. The presence of the passive oxide layer on niobium produces an ohmic contribution to the total overvoltage. The corrected activation‐overvoltage curves for niobium show a change of Tafel slope which is consistent with a Volmer‐Tafel mechanism if certain assumptions are made about the distribution of potential at the interface. Activation energies of the hydrogen reaction are computed for both rhenium and niobium electrodes.