Resistance metal-path

Resistance overpotential i/r Since in corrosion the resistance of the metallic path for charge transfer is negligible, resistance overpotential ijr is determined by factors associated with the solution or with the metal surface. Thus resistance overpotential may be defined as... 

The purpose of these installations is to provide a metal path of low resistance for the discharge of electrical currents from the air to the ground without damage to the structure or contents. These systems also serve to pre-... 

Within the electrolyte, current is carried by both negative and positive carriers, known as ions (electrically charged atoms or groups of atoms). The current carried by each ion depends on its mobUity and electric charge. The total of positive and negative current in the electrolyte of a cell is always exactly equivalent to the total current carried in the metallic path by electrons alone. Ohm s law—that is, /= E/R, where I is the current in amperes, E the potential difference in volts, and R the resistance in ohms—applies precisely, under conditions with which we are presently concerned, to current flow in electrolytes as well as in metals. 

The control of corrosion caused by stray current involves either reduction of the amount of current at the source or safe removal of the current from the corroding structure. The removal of the current from the structure involves the creation of a metallic path for the current to flow, rather than the electrolytic (soil) path. In order to create this metallic path, first the exact point of current discharge must be located through tests, and the quantity of current involved must be determined. The metallic path is provided after determining the quemtity of current that must be drained, tmd the characteristics that the drainage circuit must have. The resistance of the wire, its ampacity, and the exact point to which the current must be drained are examples of the chtuacteristics that must be determined... 

It is known that current is transported from the anode to the cathode by ions in the electrolyte and in the metallic path from the anode to cathode. Because of the normal high conductivity of metals, almost no resistance is offered to the current flow in the metallic path. However, resistance can be encountered if the distance between the anode and the cathode is appreciable. 

Stray currents are currents flowing in the electrolyte from external sources, not directly associated with the cathodic protection system. Any metallic structure, for example, a pipeline, buried in soil represents a low-resistance current path and is therefore fundamentally vulnerable to the effects of stray currents. Stray current tends to enter a buried structure in a certain location and leave it in another. It is where the current leaves the structure that severe corrosion can be expected. Corrosion damage induced by stray current effects has also been referred to as electrolysis or interference. For the study and understanding of stray current effects it is important to bear in mind that current flow in a system will not only be restricted to the lowest-resistance path but will be distributed between paths of varying resistance, as predicted by elementary circuit theory. Naturally, the current levels will tend to be highest in the paths of least resistance. 

In cathodic shielding the aim is to minimize the amount of stray current reaching the structure at risk. A metallic barrier (or shield ) that is polarized cathodically is positioned in the path of the stray current, as shown in Fig. 11.16. The shield represents a low-resistance preferred path for the stray current, thereby minimizing the flow of stray current onto the interfered-with structure. 

As fibers in the feed mat pass between the feed toU and feed plate, they ate separated by metallic wine teeth on the lickerin toU and carried to an air venturi where they ate stripped and tumbled until they strike a moving, perforated collection surface. At the collection surface, the airborne fibers foUow paths of least resistance and accumulate in a self-leveling manner while the air passes through perforations. Fiber orientation in the web is isotropic in layers corresponding to the number of fibers transferred from the wine teeth to the air-transportation 2one, the intensity of the air, and the speed of the collection surface. 

Because a length of metal associated with the connector contact is ordinarily in the path between the contact end to which a wire is terminated and the contact interface, its resistance (bulk resistance) must be added to contact resistance when considering the connector as a circuit element. This overall resistance is sometimes erroneously called contact resistance. 

The above measurements all rely on force and displacement data to evaluate adhesion and mechanical properties. As mentioned in the introduction, a very useful piece of information to have about a nanoscale contact would be its area (or radius). Since the scale of the contacts is below the optical limit, the techniques available are somewhat limited. Electrical resistance has been used in early contact studies on clean metal surfaces [62], but is limited to conducting interfaces. Recently, Enachescu et al. [63] used conductance measurements to examine adhesion in an ideally hard contact (diamond vs. tungsten carbide). In the limit of contact size below the electronic mean free path, but above that of quantized conductance, the contact area scales linearly with contact conductance. They used these measurements to demonstrate that friction was proportional to contact area, and the area vs. load data were best-fit to a DMT model. 

A convenient method of carrying out such a galvanic test in the laboratory has been described by Wesley in which the vertical circular-path machine is used. Each assembly includes two pairs of dissimilar metals—one pair coupled galvanically while the other pair is left uncoupled in order to determine the normal corrosion rates under the same environmental conditions. The type of motion provided (specimens moving in a vertical circular path) enables electrical connections to be made without mercury cup or commutator and the leads can be connected to a calibrated resistance for current measurements attached to the specimen carrier. 

Strontium nitrate [Sr(NOj)j], when burned, produces a bright red flame, and it is used in fireworks. During mihtary combat, it is used to make tracer bullets so that their paths can be tracked at night. Strontium is also used in making specialty metals when alloyed with other metals and in the manufacture of soaps, greases, and similar materials that are resistant to extreme high or low temperatures. 

The adhesives were applied according to manufacturers directions to scaled down models of an application. Failure of the vulcanized, rubber to metal bond was detected by a loss of resistance resulting from the establishment of a leak path under the rubber. The samples were connected electrically to the zinc anode. The temperature, specific gravity, electrical conductivity and pH of the solutions were monitored during the test. 

Should the force be directed so as to deflect the path of the moving charge, an additional resistance, usually called the Hall resistance, would be observed. It should be noted that the magnitude of the effect is very small, invariably much less than 1% of the initial resistance. A secondary effect occurs in magnetic metals, however, and results in a modified magnetoresistivity curve. 

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