Virtual current

Current flow through the frits is supported by ions. Cations and anions both support the virtual current by flowing in opposite directions, and the transference number of a particular ion is defined as the fraction of the total current it carries. The sum of all transference numbers then is necessarily unity. If the fraction of the virtual current carried by the cations equals the fraction carried by the anions then the solution is said to be equitransferent. 

Figure 19.29 An approximate illustration of a virtual current chopping in Phase B as a consequence of restrike of arc in Phase / ...

Fig. 5. Tentative mixed potential model for the sodium-potassium pump in biological membranes the vertical lines symbolyze the surface of the ATP-ase and at the same time the ordinate of the virtual current-voltage curves on either side resulting in different Evans-diagrams. The scale of the absolute potential difference between the ATP-ase and the solution phase is indicated in the upper left comer of the figure. On each side of the enzyme a mixed potential (= circle) between Na+, K+ and also other ions (i.e. Ca2+ ) is established, resulting in a transmembrane potential of around — 60 mV. This number is not essential it is also possible that this value is established by a passive diffusion of mainly K+-ions out of the cell at a different location. This would mean that the electric field across the cell-membranes is not uniformly distributed.

In addition to these general impedance simulations, the sensitivity of an electrode arrangement to impedance variations in the bladder region has been assessed. For this, the principle of the reciprocal lead field described by [12] and adapted to COMSOL by [11] has been used. To quantify to which extent a small volume contributes to the measured overall impedance, a second, virtual current source is introduced by making the voltage measurement electrodes current carrying. The current density vector field by this virtual current source is called the reciprocal lead field and specifies the sensitivity of the voltage measurement electrodes. The measured impedance can be expressed as... 

In the spiral-wound geometry (also called a jelly roU), the radius of curvature of the electrodes decreases as the radius of the spiral increases, thereby creating a two-dimensional geometry. Spiral-wound geometries have been treated by Evans and White [92] and Podlaha and Cheh [93]. Podlaha and Cheh describe a methodology to simulate a spiral-wound configuration with a one-dimensional model consisting of multiple ceU layers connected by "virtual" current coUectors. They compare simulations of a spiral-wound alkaline ceU to that of a bobbin cell with equivalent total reactant surface area. 

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