Resistance alternating current

Electrochemical and Electrical Methods. Electrochemical and electrical methods for studying film properties and corrosion phenomena have been extensively reviewed (29-31). Comparisons of corrosion test results with direct current measurements of conductivity suggest that visible corrosion is associated with film resistance less than about 1 Mohm/cm, but this condition may well correspond with the occurrence of virtual pores in the film allowing development of local conductive pathways. In studies of the equivalent alternating current resistance as a function of frequency, Kendig and Leidheiser (44) found that the development of a region of slope -1 on a log permittivity versus log frequency plot... 

Fio. 3. Alternating-current resistance of VaOs films in air A, 220-A film O, 1600-A film. 

The standard speaks of alternating current resistance, whereas electrotechnicians know that this relation gives the modulus of the impedance. 

In high-temperature superconductors, the BCS theory does not seem to apply. A currently in-fashion idea is that high-temperature superconductors are the first d wave. For alternating current, resistance increases slowly with increasing frequency. However, even at microwave frequencies, the resistance of a superconductor is approximately 1/1000 the resistance of copper or silver. 

Induction furnaces utilize the phenomena of electromagnetic induction to produce an electric current in the load or workpiece. This current is a result of a varying magnetic field created by an alternating current in a cod that typically surrounds the workpiece. Power to heat the load results from the passage of the electric current through the resistance of the load. Physical contact between the electric system and the material to be heated is not essential and is usually avoided. Nonconducting materials cannot be heated directiy by induction fields. 

Rhodium and iridium have a resistance to anodic corrosion comparable with that of platinum, and are more resistant to the influence of alternating currents. A platinum-iridium alloy, in the form of a coating on titanium, is preferred to pure platinum for the production of chlorine from brine , due to its improved corrosion resistance and lower overvoltage. 

Valve voltmeters were widely used in the past, but have been replaced by transistor voltmeters. With instruments of this type it is possible to achieve an input resistance of 50 MQ or more, the current required to operate the instrument being of the order of 10" A. The early instruments had a tendency to zero drift on the lower ranges, but this has been overcome in the modern transistor types. Such instruments are most often used to make potential readings in extremely high-resistance electrolytes. The accuracy of such instruments is of the order of 2% full-scale deflection. It is necessary to ensure that both types are so designed that they do not respond to alternating currents. 

To avoid the errors of polarisation and stray currents, special resistivity meters are employed. One form of these uses an alternating current produced from batteries by a vibrator. The effective resistance is measured by a modified Wheatstone bridge with balance indicated by a galvanometer. 

Hard anodic films, 50-100/rm thick, for resistance to abrasion and wear under conditions of slow-speed sliding, can be produced in sulphuric acid electrolytes at high current density and low temperature. Current densities range from 250 to 1 000 Am , with or without superposed alternating current in 20-100g/1 sulphuric acid at —4—I- 10°C. Under these conditions, special attention must be paid to the contact points to the article under treatment, in order to avoid local overheating. 

The hardness of the film is markedly affected by the conditions of anodising. By means of special methods involving dilute electrolytes at low temperatures and relatively high voltages , with or without superimposed alternating current, it is possible to produce compact abrasion-resistant films with thicknesses of 50-75/im and hardnesses of 200-500 VPN, for special applications. 

As already indicated conductimetric measurements are normally made with alternating current of frequency 103Hz, and this leads to the existence of capacitance as well as resistance in the conductivity cell. If the frequency of the current is increased further to 106 — 107 Hz, the capacitance effect becomes even more marked, and the normal conductivity meter is no longer suitable for measuring the conductance. 

The conclusion regarding the fact that constant current conductivity involves not all microcrystals of the sample is proved by results of measurements of electric conductivity in sintered ZnO films in case of alternating current (Fig. 2.10). The availability of barrier-free ohmic pathways is proved by a low value of initial resistivity in sintered samples ( 1 - 5 kOhm) in addition to exponential dependence of electric conductivity plotted as a function of inverse temperature having activation energy 0.03 - 0.5 eV, which coincides with ionization energy of shallow dope levels. The same value is obtained from measurements of the temperature dependence of the Hall constant [46]. 

Electrolytes, like electronic conductors, obey Ohm s Law, except under such extreme conditions as those involving very high voltages and/or very high-frequency alternating currents. The conductance of an electrolyte is calculated from measurements of its resistance. 

Apart from use in metallurgical research and measurements, solid electrolytes have also been put to use as heating elements in electrical resistance furnaces. In order to prevent electrolysis from occurring, alternating currents must be used. In contrast to metallic heating elements, they may be used in air at around 2000 °C. In view of the fact that their conductivity must be very low at room temperature, there is a need for them either to be kept continuously warm, or to be preheated with an auxiliary heating element. 

The conductivity of an ionic conductor can be assessed by direct current (dc) or alternating current (ac) methods. Direct current methods give the resistance R and the capacitance C. The corresponding physical quantity when ac is applied is the impedance, Z, which is the total opposition to the flow of the current. The unit of impedance is the ohm (fl). The impedance is a function of the frequency of the applied current and is sometimes written Z(to) to emphasize this point. Impedance is expressed as a complex quantity ... 

Electrical resistance is a broad term given to the opposition of flow of current within an electrical circuit. However, when considering components such as capacitors or inductors, or when speaking about resistance to alternating current (AC) flow, certain other terminology is used. 

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