Physical characterization electrical measurements

In many cases, we will regard our biological material, together with the necessary electrode arrangements, as an unknown black box. By electrical measurement, we want to characterize the content of the box (we do not have direct access to the key to open the lid ). We want to use the data to describe the electrical behavior, and perhaps even explain some of the physical or chemical processes going on in the box, and perhaps discern between electrode and tissue contributions. The description must necessarily be based upon some form of model (e.g., in the form of an equivalent electric circuit), mimicking measured electric behavior. We may also want to link properties to distinct tissue parts or organ parenchyma behavior. A basic problem is that always more than one model fits reasonably the measured electric behavior. 

L. R. Weisberg, Electrical measurement for trace characterization, in Trace Characterization, Chemical and Physical (W. W. Meinke and B. F. Scribner, eds.), NBS Monograph 100, U.S. Government Printing Office, Washington, D.C. (1967). 

The importance of the characterization of the ambient, in terms of the measurements of those parameters capable of describing and/or controlling the ambient, is commonly recognized. These measurements can be done by special devices able to sense the physical or chemical quantities and possibly give an electric output signal. 

Seawater contains about 3.5% salts, in which the content of sodium chloride is about 80%. The concentration of dissolved salts as well as temperature and pressure influence the physical properties of seawater. The total salt concentration is usually called salinity . Salinity is generally measured by the electrical conductivity or determination of chloride content. At present, salinity(S) is defined as S = 1.80655 Cl (Cl is the concentration of chloride in seawater) [5]. Dissolved oxygen and silica are usually measured as additional parameters to characterize seawater. The concentrations of nitrogen and phosphorus are the indices of nutrients and measure the fertility and production of the oceans. 

It is not difficult to observe that in all of these expressions we have a multiplication between the property gradient and a constant that characterizes the medium in which the transport occurs. As a consequence, with the introduction of a transformation coefficient we can simulate, for example, the momentum flow, the heat flow or species flow by measuring only the electric current flow. So, when we have the solution of one precise transport property, we can extend it to all the cases that present an analogous physical and mathematical description. Analogous computers [1.27] have been developed on this principle. The analogous computers, able to simulate mechanical, hydraulic and electric micro-laboratory plants, have been experimented with and used successfully to simulate heat [1.28] and mass [1.29] transport. 

This section will focus mainly on characterization of the physical nature of the dispersed phase or its size distribution. Electrokinetic characterization techniques, which determine the electric double-layer properties of the dispersed phase, will be only briefly mentioned. Again, electrokinetic properties, their significance, and their measurement have been covered in review articles 44, 45). 

In order to select a particular experimental technique to measure x , it is very important to keep in mind which parameter of the third-order nonlinear response has to be characterized. For example, if one wants to determine the time-response due to molecular reorientation, one cannot choose Third-Harmonic Generation or Electric-Field-Induced Second-Harmonic Generation, since none of these techniques provide time-response information. Depending on the parameter of interest, a specific technique must be chosen. The following physical mechanisms can contribute to the third-order nonlinear response [54] ... 

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