Voltage measurement

Figure 3-1 Voltage Measurements on a Silver-Silver Chloride, Hydrogen Cell at 298.15 K. The contribution of the Standard Hydrogen Electrode is taken as zero by convention.

Voltage measurement have been made at very low temperatures using a superconductor as one leg of a thermocouple. Eor a superconductor, S is zero, so the output of the couple is entirely from the active leg. The Thomson heat is then measured at higher temperatures to extend the absolute values of the Seebeck coefficients (7,8). The Thomson heat is generally an order of magnitude less than the Peltier heat and is often neglected in device design calculations. 

Fig. 2. Measuring static charge, (a) Field voltage measurement (b) Faraday cage (c) surface resistivity measurement and (d) static decay test.

In the thermodynamic treatment of electrode potentials, the assumption was made that the reactions were reversible, which implies that the reactions occur infinitely slowly. This is never the case in practice. When a battery deUvers current, the electrode reactions depart from reversible behavior and the battery voltage decreases from its open circuit or equiUbrium voltage E. Thus the voltage during battery use or discharge E is lower than the voltage measured under open circuit or reversible conditions E by a quantity called the polari2ation Tj. 

Flow is an important measurement whose calibration presents some challenges. When a flow measurement device is used in applications such as custody transfer, provision is made to pass a known flow through the meter. However, such a provision is costly and is not available for most in-process flowmeters. Without such a provision, a true cahbration of the flow element itself is not possible. For orifice meters, calibration of the flowmeter normally involves cahbration of the differential pressure transmitter, and the orifice plate is usually only inspected for deformation, abrasion, and so on. Similarly, cahbration of a magnetic flowmeter normally involves cahbration of the voltage measurement circuitry, which is analogous to calibration of the differential pressure transmitter for an orifice meter. 

This is not material in voltage transformers, as neither the voltage measuring instruments nor the protective relays will carry any inrush current during a switching operation or a fault. No short-time rating is thus assigned to such transformers. 

In all electrical measurements, current and voltage measuring instruments with two terminals are employed. The object being measured similarly has two termi-... 

Fig. 3-2 Current and voltage measurement in a current-voltage diagram (explanation in the text).

Resistance is measured either indirectly by separate current and voltage measurements or directly by comparison in a bridge. In both cases it involves two measurements. The instruments for measuring the current and voltage U2 have to be chosen so as to make the deviation from U and I in Fig. 3-2 as small as possible. 

Fig. 9./-Tbias curve of an individual semiconducting SWCNT with different gale voltages measured at room temperature [29].

Where V and I refer to full-load current and rated voltage and IZ is the voltage measured at rated current during a short-circuit test transformer. In the case of a transformer... 

An important point to be considered when the instrument is used for A.C. voltage measurement is the terminal connections. One terminal will be clearly designated as the high-potential connection, and this should be adhered to. The HT terminal will have a low value of capacitance to other bodies and to earth while the corresponding capacitance of the other is high. If the instrument is in a metallic case this should be connected to the mains earth as a safety precaution. In some cases, the low-voltage terminal is also connected to the metallic case. If this is so, the instrument will effectively earth the circuit under test, which may give rise to problems. 

One of the most important characteristics of a cell is its voltage, which is a measure of reaction spontaneity. Cell voltages depend on the nature of the half-reactions occurring at the electrodes (Section 18.2) and on the concentrations of species involved (Section 18.4). From the voltage measured at standard concentrations, it is possible to calculate the standard free energy change and the equilibrium constant (Section 18.3) of the reaction involved. 

Schematic energy level diagrams of a metal/polymer/metal structure before and after the layers are in contact are shown in the top two drawings of Figure 11-6. Before contact, the metals and the polymer have relative energies determined by the metal work functions and the electron affinity and ionization potential of the polymer. After contact there is a built-in electric field in the structure due to the different Schottky energy barriers of the asymmetric metal contacts. Capacitance-voltage measurements demonstrate that the metal/polymer/metal structures are fully depleted and therefore the electric field is constant throughout the bulk of the structure [31, 35]. The built-in potential, Vhh i.e. the product of the constant built-in electric field and the layer thickness may be written...

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

We would like to measure the contribution each half-reaction makes to the voltage of a cell. Yet every cell involves two half-reactions and every cell voltage measures a difference between their half-cell potentials. We can never isolate one half-reaction to measure its E°. An easy escape is to assign an arbitrary value to the potential of some selected half-reaction. Then we can combine all other half-reactions in turn with this reference half-reaction and find values for them relative to our reference. The handiest arbitrary value to assign is zero and chemists have decided to give it to the half-reaction... 

During charging and discharging of the cell, the terminal voltage U is measured between the poles. It should also be possible to calculate directly the thermodynamic terminal voltage from the thermodynamic data of the cell reaction. This value often differs slightly from the terminal voltage measured between the poles of the cell because of an inhibited equilibrium state or side reactions. 

Randomness.—The word random is used frequently to describe erratic and apparently unpredictable variations of an observed quantity. The noise voltage measured at the terminals of a hot resistor, the amplitude of a radar signal that has been reflected from the surface of the sea, and the velocity measured at some point in a turbulent air flow are all examples of random or unpredictable phenomena. 

The problem just considered can be generalized in a useful way by assuming that we want to predict the value of a time function Y at time t from our knowledge of the value of a different time function X at time t. For example, X(t) could be a noise voltage measured at some point in an electrical network and F(f) the noise voltage measured at a... 

A voltmeter joined between the two electrodes of a galvanic cell shows a characteristic voltage, which depends on the concentration and nature of participating reactants. For example, in the Cu-Zn cell, if Cu2+ and Zn2+ are at 1 mol dm-3 (1 M) concentrations and the temperature is 298 K, the voltage measured would be 1.10 V. This voltage is characteristic of the reaction as shown below ... 

The potential of a half-reaction is a measure of the disposition of that half-reaction to take place, no matter what the other half of the complete reaction is. Thus, the potential of any complete reaction can be obtained by adding potentials of its two half-reactions. The potential so obtained is a measure of disposition of the complete reaction to occur, and provides the voltage measured for a galvanic cell which was the overall reaction. For example, the entries in Table 6.11 for Ni and Ag electrodes are ... 

The cell voltage measurement in itself represents a point of decisive significance, where factors such as temperature of the measurement, and Nemstian behaviour and asymmetry of the electrode play a role together with the reliability and flexibility of the pH/mV meter. Such a meter consists of a null-point or a direct-reading meter. 

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