Electrical resistivity change under

An electrical resistance methods which directly measures loss of metal from a probe installed in the corrosive system under study is described in Section 19.3. It is reported that corrosion equivalent to a thickness loss of as little as 2-5 X 10 cm can be detected . This technique is most useful as a means of monitoring steps taken to reduce corrosion, e.g. by inhibitors, or to detect changes in the corrosivity of process streams. Electrical methods of determining corrosion rates are considered subsequently. 

In terms of environmental exposure, water and humidity must be carefully evaluated in electrical applications. In general, if a plastic absorbs a significant amount of water, the electrical resistivity drops. As examples this is the case for nylons and phenolic. Care must be used in selecting a dielectric to insure that the electrical properties such as the insulation resistance and dielectric strength, as well as other electrical properties are adequate under the conditions of field use, particularly if this involves exposure to high humidity conditions. Temperature also causes changes in most electrical products. 

In addition to high permselectivity, the membrane must have low-electrical resistance. That means it is conductive to counterions and does not unduly restrict their passage. Physical and chemical stability are also required. Membranes must be mechanically strong and robust, they must not swell or shrink appreciably as ionic strength changes, and they must not wrinkle or deform under thermal stress. In the course of normal use, membranes m be expected to encounter the gamut of pH, so they should be stable from 0 < pH < 14 and in the presence of oxidants. 

Fig. 5.6. Changes in electrical resistance of ZnO film under the action of singlet oxygen at its chemical generation [66]. Arrow-heads indicate on/off switching...

Figure 9.3 Changes in transepithelial electrical resistance (TEER) of human nasal epithelial cell layers grown under LCC ( ) versus AIC (A) conditions. Each data point represents the mean SD of three determinations. (Data from Ref. [46]).

This transition is accompanied by a change in the resistivity of powders, which is measured by means of pressure contacts (Fig. 2b). The total change in resistivity is equal to six orders of magnitude in the density range under investigation and corresponds to a transition from the dielectric state to the metal one. The critical density, i.e., the density at which the transition occurs, is equal to ps2.7-2.9 g/cm according both to diffraction measurements and to electric-resistivity measurements. 

The electric resistance of pure iron increases from 0° C. to a maximum at 757° C., corresponding to the A2 point, and then falls to a minimum at 894° C.—the A3 point. On cooling the reverse changes occur at practically the same temperatures.4 The presence of hydrogen under atmospheric pressure does not materially affect the resistance of the metal up to 920° C.5... 

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