Variable impedance

These bridges, as shown in Fig. 11.1, use the principle of balance between the electrochemical cell under study and a variable impedance, Zs, which in research on electrode processes normally consists of a resistance Rs in series with a capacitance Cs. Given that... 

In general, two adjustable elements are sufficient to satisfy relations (16) and (17). In practice, it is desirable to have the adjustments in the resistances independent of those in the reactances. This can be achieved in a ratio bridged Let us put the unknown element in arm 1 of the bridge, allow arm 3 to have a variable impedance Z3, and choose the ratio arms 2 and 4 so that Z2/Z4 = K, where Kis some real positive number (i.e., RilR = Z2/Z4 = K). This is equivalent to having the same phase shift in legs 2 and 4, i.e., 2 = tpi It follows from Eq. (13) that the balance condition for this ratio bridge is... 

A calorimetric method was also used by Susz 60), but was so tedious that he abandoned it for the three ammeter method without, however, citing data to compare the results of the two methods. The data he cites for ozone synthesis appear to relate to power measured by the latter method, which gives erroneous values for any load of variable impedance such as an ozonizer discharge for the most part they relate to... 

When doing a split-half experiment it has to be decided which mode to split. Splitting must be performed in a mode with a sufficient number of disjoint variables/samples, in such a way that each subset will span the whole domain and hence that all the parameters can be estimated from the individual subsets. For example, the conditions for uniqueness must apply to all the subsets. With a high-dimensional spectral mode, an obvious idea is to use this spectral mode for splitting, but the collinearity of the variables impedes sound results and split-half analysis is meaningless. Therefore, it is most common to split the data in the sample mode. 

A fixed resistor R especially for low frequencies f suffers several limitations. Therefore, the resistor R is replaced by an amplifier with a variable gain according to Fig. 12.7. This results in a variable impedance Zx(ffl) which can be adjusted to the impedance of the sample Zs(m). For the sample impedance, then... 

Variable impedance A capacitor, inductor, or resistor that is adjustable in value. 

The same transducer structure can serve as either an actuator or a generator, depending on how it is electrically driven. Therefore, the relationship between motion and energy of deformation can be modulated. In other words, the transducer can be a variable impedance device in which stiffness and damping are electrically controllable. Since the same transducer structure can serve as an actuator, generator, sensor or variable impedance device, the transducer can be said to be multifunctional. The use of dielectric elastomers as variable stiffness devices is discussed in detail elsewhere [9]. 

Blaya, J. and Herr, H., Adaptive Control of a Variable-Impedance Ankle-Foot Orthosis to Assist Drop-Foot Gait, IEEE Trans. Neural Systems Rehabilitation Engineering, 12(1), 24—31,2004. 

The proposed measurement system consist of measuring impedance variations, resistance and inductance, using a coil surrounding a cylindrical hallow sample, in with standard defects were created homy a variable length and width... 

In another type of measurement, the parallel between mechanical and electrical networks can be exploited by using variable capacitors and resistors to balance the impedance of the transducer circuit. These electrical measurements readily lend themselves to computer interfacing for data acquisition and analysis. 

Fig. 1.20 Cell consisting of two reversible Ag /Ag electrodes (Ag in AgN03 solution). The rate and direction of charge transfer is indicated by the length and arrow-head as follows gain of electrons by Ag -he- Ag—> loss of electrons by Ag - Ag + e- —. (o) Both electrodes at equilibrium and (f>) electrodes polarised by an external source of e.m.f. the position of the electrodes in the vertical direction indicates the potential change. (K, high-impedance voltmeter A, ammeter R, variable resistance)...

The hot-ER test seems fairly reliable in indicating the temperature at which the impedance rises, but shows some variability in characterizing the subsequent drop in impedance. 

In (photo)electrochemistry, the expected photocurrent change, Ai, is typically dependent in a nonlinear way on the changes in the potential applied. The reciprocal complex impedance, 1/AZ, is the variable. The real part (flAconductivity change across the elec-... 

The ability to change and control the composition of the nutrient solution and the relatively small size of the microcosms used enables manipulation of environmental variables and time-course studies of rhizodeposition to be made relatively easily. The influence of nutrient availability, mechanical impedance, pH, water availability, temperature, anoxia, light intensity, CO2 concentration, and microorganisms have all been examined within a range of plant species (9). A few examples to illustrate the continued interest in examining the effect of such variables on rhizodeposition in nutrient culture are given in Table 1. 

To do this, not only must he know the chemistry of the reactions but he must know the rates at which the reactions occur and what affects those rates. The study of this is called chemical kinetics. By the proper choice of raw materials and operating conditions for the reaction stage the process designer can manipulate the ratio of products formed. One major variable is the temperature. An increase in temperature usually causes the reaction rates to increase, but some increase faster than others. Thus, the product mix in the reactor is dependent on the temperature. The pressure and the time the material spends in the reactor also affects the results. In the gaseous phase ahigh pressure will impede those steps in which the number of moles is increased and assist those in which the number of moles is decreased. A... 

In the end, analysis of vs 0 (the "classical" approach) is not quantitative, a problem we associate with surface area variability and ambiguity arising from the interpretation of P . To help determine which adsorption mechanism is operative, we turn to an alternative parameter, the IS impedance "time constant", T, which does not suffer from these drawbacks. 

Very often, the electrode-solution interface can be represented by an equivalent circuit, as shown in Fig. 5.10, where Rs denotes the ohmic resistance of the electrolyte solution, Cdl, the double layer capacitance, Rct the charge (or electron) transfer resistance that exists if a redox probe is present in the electrolyte solution, and Zw the Warburg impedance arising from the diffusion of redox probe ions from the bulk electrolyte to the electrode interface. Note that both Rs and Zw represent bulk properties and are not expected to be affected by an immunocomplex structure on an electrode surface. On the other hand, Cdl and Rct depend on the dielectric and insulating properties of the electrode-electrolyte solution interface. For example, for an electrode surface immobilized with an immunocomplex, the double layer capacitance would consist of a constant capacitance of the bare electrode (Cbare) and a variable capacitance arising from the immunocomplex structure (Cimmun), expressed as in Eq. (4). 

The third block in Fig. 2.1 shows the various possible sensing modes. The basic operation mode of a micromachined metal-oxide sensor is the measurement of the resistance or impedance [69] of the sensitive layer at constant temperature. A well-known problem of metal-oxide-based sensors is their lack of selectivity. Additional information on the interaction of analyte and sensitive layer may lead to better gas discrimination. Micromachined sensors exhibit a low thermal time constant, which can be used to advantage by applying temperature-modulation techniques. The gas/oxide interaction characteristics and dynamics are observable in the measured sensor resistance. Various temperature modulation methods have been explored. The first method relies on a train of rectangular temperature pulses at variable temperature step heights [70-72]. This method was further developed to find optimized modulation curves [73]. Sinusoidal temperature modulation also has been applied, and the data were evaluated by Fourier transformation [75]. Another idea included the simultaneous measurement of the resistive and calorimetric microhotplate response by additionally monitoring the change in the heater resistance upon gas exposure [74-76]. 

The cell plasma membrane separates the cell cytoplasm from the external medium. The composition of the cytoplasm must be tightly controlled to optimize cellular processes, but the composition of the external medium is highly variable. The membrane is hydrophobic and impedes solute diffusion. But it also facilitates and regulates solute transfers as the cell absorbs nutrients, expels wastes and maintains turgour. 

Shock is a clinical syndrome in which profound and widespread reduction in the effective delivery of oxygen and other nutrients to the tissues. In shock condition, the individual is weak, anxious with coldness of extremeties, sweeting and marked fall in arterial pressure. Physiologic mechanisms can effect the arterial pressure by acting on one or more of two variables i.e. preload, impedance to blood flow (after load) and myocardral contractility. These macha-nisms include ... 

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