Local current flow

Signal travels by local current flow or by saltatory conduction... 

Saltatory conduction results in a significant increase in the velocity of conduction of the nerve impulse down the axon compared to that of local current flow in an unmyelinated axon (see Table 4.2). The speed of conduction is... 

Figure 9 is a diagram of the local currents flowing along the nerve fiber during passage of an excitation impulse. To review the excitation propagation mechanism described in Section 1 it is convenient to consider Figure 9. The downstream-oriented axial current inside the fiber crosses the membrane as capacitive current, i.e., charges the membrane. When a certain threshold is exceeded, an inward ionic current appears apart from capacitive current. Further on, the outward current begins to dominate and returns the potential...

According to tidal current data observed during August 2008, local current flowed as the... 

The noble metals in saline solution are highly polarizable, and the DC voltage wiU be unstable and poorly defined unless under strict zero current conditions. Metal aUoy inhomogeneity will create noise as a function of local current flow between different parts of the metal surface. The noise generation of stainless steel electrodes is very dependent on the alloy composition (see Section 7.3). 

In electrochemistry, an electrolytic cell is said to be polarized if a current flows through the cell. A CC electrode is therefore a polarized electrode. Recording electrodes (PU electrodes) are in general not considered to be polarized. However, a metal electrode may be positioned in a region that is not isoelectric, or the metal surface properties may vary. The result is local current flow and endogenic polarization without any current in the external electrode wire. 

Fig. 9.1 Model of a cylindrical pore gray area is not conductive. / axial current flowing to pore, i local current flowing to pore walls...

Undesired side reactions occur under starvation conditions, as discussed in detail in Section 20.4. These cell states are caused by an inhomogeneous distribution of reactant gases, temperature, and local water content across the active area during fuel-cell operation. Measurement of the current density distribution allows the detection of these critical conditions. However, gradients of the electrochemical reaction rates are mostly compensated by in-plane currents within the catalyst layer, gas diffusion layer, and bipolar plates. Therefore, for measurement of the current density distribution, the local current flow has to be separated into individual pathways. 

Fig. 6.13 A broad tip in a STM (A) may change the position where the localized current flows as it moves across a rough surface. The result is to smooth out the apparent surface. A sharp pyramidal tip (B) follows the surface much more accurately. (From Nishikawa et al [137] reproduced with permission.)...

Subsequent theoretical studies have suggested that the form of coarse mesh diffusion theory used in the standard applications of the AIMAZ code does not represent the high local current flows from relatively highly enriched fuel to a highly borated reflector with sufficient accuracy. The problem is, however, much less significant at the lower boron levels arising at power operation. 

In contrast to a direct injection of dc or ac currents in the sample to be tested, the induction of eddy currents by an external excitation coil generates a locally limited current distribution. Since no electrical connection to the sample is required, eddy current NDE is easier to use from a practical point of view, however, the choice of the optimum measurement parameters, like e.g. the excitation frequency, is more critical. Furthermore, the calculation of the current flow in the sample from the measured field distribution tends to be more difficult than in case of a direct current injection. A homogenous field distribution produced by e.g. direct current injection or a sheet inducer [1] allows one to estimate more easily the defect geometry. However, for the detection of technically relevant cracks, these methods do not seem to be easily applicable and sensitive enough, especially in the case of deep lying and small cracks. 

This simplified discussion has neglected the effects of axial current flow, ie. Hall current, induced by the axial field. At a local region in the channel, equation 1 can be written in more general form as... 

Charge Transport. Side reactions can occur if the current distribution (electrode potential) along an electrode is not uniform. The side reactions can take the form of unwanted by-product formation or localized corrosion of the electrode. The problem of current distribution is addressed by the analysis of charge transport ia cell design. The path of current flow ia a cell is dependent on cell geometry, activation overpotential, concentration overpotential, and conductivity of the electrolyte and electrodes. Three types of current distribution can be described (48) when these factors are analyzed, a nontrivial exercise even for simple geometries (11). 

This occurs whcti two dissimilar metals in an electrolyte have a tnelallic tie between them. There is a How of electricity between the anodic and cathodic metal surfaces, generated by the local cells set between dissimilar mentis. One metal becomes an anode and the other a cathode and causes an anodic reactioti w hieh represents acquisition of charges by the corroding melal. The anode corrodes tind protects the cathode, as current flows through the electrolyte between them. 

Figure 1.62b shows the result of raising the potential of a corroding metal. As the potential is raised above B, the current/potential relationship is defined by the line BD, the continuation of the local cell anodic polarisation curve, AB. The corrosion rate of an anodically polarised metal can very seldom be related quantitatively by Faraday s law to the external current flowing, Instead, the measured corrosion rate will usually exceed... 

Semiconducting devices, switches and miniaturised v.h.f. circuits are all particularly sensitive to the slightest reaction on critical surfaces, and in devices calling for the highest levels of reliability even the most inert of the phenolic, epoxide and silicone resins are not considered to be fully acceptablecorrosion of electronic assemblies may often be enhanced by migration of ions to sensitive areas under applied potentials, and by local heating effects associated with current flows. 

Voltage-gated potassium (Kv) channels are membrane-inserted protein complexes, which form potassium-selective pores that are gated by changes in the potential across the membrane. The potassium current flow through the open channel follows by the electrochemical gradient as defined by the Nernst equation. In general, Kv channels are localized in the plasma membrane. 

---