Measurement of current

One should select a lower secondary current, say, 1 A CT, for installations requiring long lengths of connecting leads, such as for remote measurement of current or other quantities. It is advisable to limit the extra VA burden on the CTs, on account of such leads. 

A further measurement of current can be obtained from the pipe mass per meter, given in the standards, m = mil = S... 

In making measurements of current flowing within a structure, it is extremely important that additional resistance, as for example a shunt, is not introduced into the circuit, as otherwise erroneous results will be obtained. One method is to use a tong test meter. Such instruments are, however, not particularly accurate, especially at low currents, and are obviously jmpracticablein thecaseof, say, a 750 mm diameter pipeline. A far moreaccurate method and onethat can beapplied to ail structures, isthe zero-resistance ammeter or, as it is sometimes called, the zero-current ammeter method. The basic circuit of such an instrument is shown in Fig. 10.47. 

The measurement of current densities in the vicinity of a cathodically protected structure is a comparatively new principle used chiefly to monitor the effectiveness of offshore protection systems. These measurements are undertaken by twin half-cell devices either installed for stationary use or moved about the structure by diver or remote controlled vehicle. 

Measurements of current using same-metal electrodes are utilised for electrochemical noise measurements see section below). 

Electrical methods of analysis (apart from electrogravimetry referred to above) involve the measurement of current, voltage or resistance in relation to the concentration of a certain species in solution. Techniques which can be included under this general heading are (i) voltammetry (measurement of current at a micro-electrode at a specified voltage) (ii) coulometry (measurement of current and time needed to complete an electrochemical reaction or to generate sufficient material to react completely with a specified reagent) (iii) potentiometry (measurement of the potential of an electrode in equilibrium with an ion to be determined) (iv) conductimetry (measurement of the electrical conductivity of a solution). 

Amperometry refers to measurement of current under a constant applied voltage and under these conditions it is the concentration of the analyte which determines the magnitude of the current. Such measurements may be used to follow the change in concentration of a given ion during a titration, and thus to fix the end point this procedure is referred to as amperometric titration. 

The field sometimes called molecular electronics actually should extend well beyond simple measurement of current/voltage characteristics of single molecules. The latter topic, single molecule transport, has comprised by far the dominant reported molecular electronics measurement and modeling, and, as has been discussed above, the community is reaching some agreement in this area. 

An early measurement of current through a molecule was the report in 1995 of the resistance of a single C60 molecule, 1 (Fig. 6), deposited on an Au substrate and located and measured by an STM probe [59]. The conductivity was a respectable 18 nS. Most molecules studied as wires are more linear, with a coordinating atom at one or both ends. 

Fig. 3 Principle of electrolyte gating. Tuning of the Fermi levels of WEI and WE2 relative to the molecular levels enables measuring of current (0-voltage (E) characteristics i vs ( wei -L we2) at fixed wei or we2, i vs wei or we2 at fixed bias Ebias = ( wei -Ewe2> as well as barrier height profiles i vs distance z of tailored molecular junctions in a vertical SPM-based configuration respective horizontal nanoelectrode assembly...

The classical electrochemical methods are based on the simultaneous measurement of current and electrode potential. In simple cases the measured current is proportional to the rate of an electrochemical reaction. However, generally the concentrations of the reacting species at the interface are different from those in the bulk, since they are depleted or accumulated during the course of the reaction. So one must determine the interfacial concentrations. There axe two principal ways of doing this. In the first class of methods one of the two variables, either the potential or the current, is kept constant or varied in a simple manner, the other variable is measured, and the surface concentrations are calculated by solving the transport equations under the conditions applied. In the simplest variant the overpotential or the current is stepped from zero to a constant value the transient of the other variable is recorded and extrapolated back to the time at which the step was applied, when the interfacial concentrations were not yet depleted. In the other class of method the transport of the reacting species is enhanced by convection. If the geometry of the system is sufficiently simple, the mass transport equations can be solved, and the surface concentrations calculated. 

The measurement of current and potential provides no direct information about the microscopic structure of the interface, though a clever experimentalist may make some inferences. During the past 20 years a number of new techniques have been developed that allow a direct study of the interface. This has led to substantial progress in our understanding of electrochemical systems, and much more is expected in the future. We will review the principles of several of these techniques in Chapter 15. Many of them are variants of spectroscopies familiar from other fields. 

The traditional electrochemical techniques are based on the measurement of current and potential, and, in the case of liquid electrodes, of the surface tension. While such measurements can be very precise, they give no direct information on the microscopic structure of the electrochemical interface. In this chapter we treat several methods which can provide such information. None of them is endemic to electrochemistry they are mostly skillful adaptations of techniques developed in other branches of physics and chemistry. 

FIGURE 6.8 A simple circuit drawn using the symbols for voltage and resistance. The measurement of current is illustrated with the arrow inside the circle. 

Polarographic me liods involve the measurement of current as the voltage applied is gradually increased. 

Amperometric methods involve the measurement of current t-iat results trom the application ot a fixed voltngi. ... 

The term chronoamperometry means the measurement of currents as a function of time, and can be thought of as a kind of controlled-potential voltammetry or controlled-potential microelectrolysis (in unstirred solution) . 

Nomenclature. The form of words we employ in electroanalysis will tell us much about the parameters under study, as shown by Table 1.2. Being able to take a word apart, bit by bit, will tell us what the overall electroanalytical term means. We have already mentioned potentiometry and amperometry as being techniques for following potential and current, respectively. As another example, there is a commonly used joint term, i.e. voltam- , which implies measurement of current in response to potential variation. An example of this is a voltammogram, which is a trace ( -gram ) of current ( ammo- ) as a function of potential ( volt- ). 

Measurement of current. In order to measure a current, we must use an ammeter, or any device capable of acting as an ammeter. (Remember the root amm- will always mean current here.)... 

We are now in a position to see the major difference between measurement of potential and measurement of current. We recall that we want a zero current when measuring the potential. We see from the argument above that a zero current implies that no compositional changes occur inside an electrochemical cell. Conversely, compositional changes do occur during measurement of current, precisely because charge is transferred. 

In summary, then, the biggest difference between measurements of potential and current (potentiometry and amperometry, respectively) is that potentials are measured with a zero current wherever possible, implying that no compositional changes occur inside the cell during measurement, whereas compositional changes do occur during the measurement of current. 

Molar conductivity measurements are equally applicable to both solid and liquid electrolytes. In contrast, the measurement of current flowing through an electrochemical cell on a time scale of minutes or hours while the cell is perturbed by a constant dc potential is only of value for solid solvents (Bruce and Vincent, 1987) where convection is absent. Because of the unique aspects of dc polarisation in a solid solvent this topic is treated in some detail in this chapter. Let us begin by considering a cell of the form ... 

We are indebted to Dr. D. Snelling for the performance measurements of current voltage and power shown in Figure U, and for other relevant data. 

Voltammetry is the second most utilized technique for electronic tongue devices (see Fig. 2.6). It is a d)mamic electroanalytical method, that is, a current flow passes through the measurement cell (z 0). Voltammetry consists of the measurement of current at a controlled potential constant or, more frequently, varying. In the classic three-electrode cell configuration, the current flows between two electrodes, called working and counter (or auxiliary) respectively, while the potential is controlled between the working and a third electrode, the reference (Kissinger and Heineman, 1996). 

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