Changes of resistivity

The measurement range for platinum is -200 to +800 °C, for nickel -50 to +250 °C, and for copper -50 to +200 °C. The advantages are good accuracy, almost linear characteristics, and stability. A disadvantage is the small change of resistance with temperature, which requires a high sensitivity from the rest of the measurement equipment. 

Figure 2 Relative change of resistivity during isochronal annealing (AT=10K, At=10min) of deformed samples Deformed in disordered state 40% (A) and 80% reduction ( ) deformed in ordered state 30% reduction ( )...

Although there have been rather early hints at the correlation between changes of resistivity and ordering the fundamental work of Rossiter based on the pseudo-potential formalism was of most importance for experimental research. 

The direct proof of hydride formation in situ in a reaction vessel is in principle possible. One can follow changes of resistance (of a film, a wire, etc.) or of magnetic susceptibility of a catalyst. Hydride identification by means of the X-ray diffraction method requires a catalyst sample to be taken out from a reaction vessel, and eventually frozen in order to avoid a rapid decomposition of the hydride under ambient conditions (67). 

We heated the substrate of zinc oxide containing 10 cm 2 of silver atoms (in this case there was already no emission after completion of deposition) at 300 C. Such thermal treatment results in formation of microcrystals, rather than evaporation adatoms on the surface of the substrate made of zinc oxide. In paper [34] it was shown that microcrystals with diameter 100 A deposited on the zinc oxide surface are acceptors of electrons, therefore the formation of microcrystals results in increase of resistivity of a sensor substrate above the initial value (prior to silver deposition). In this case the initial value of the resistance of sensor-substrate was 2.1 MOhm, after adsorption of silver atoms it became 700 kOhm, and as a result of heating at 300°C and formation of microcrystals - acceptors of electrons it in increased up to 12 MOhm. If such a substrate is subject to deposition of 3-10 5 cjjj-2 silver again, then emission of silver atoms gets detected. From the change of resistivity of sensor-detector due to deposition of silver atoms one can conclude that in this case the emission of atoms is 4 times as low than in experiment with pure substrate made of zinc oxide, which confirms the supposition made on the mechanism of emission of adatoms. 

The magnetoresistance of a solid is the change of resistance when a magnetic field is applied. The magnetoresitive (MR) ratio is defined as the change in resistance when a magnetic field is applied to that in zero field ... 

Sachtler and Dorgelo (96a,b) observed that the direction of change of resistance, when hydrogen is adsorbed on a nickel film, depends on the conditions under which the film is prepared. With films prepared by evaporation under very high vacuum (< 10 mm.), the resistance decreases, while films evaporated under conditions of lower vacuum ( 10 mm.) show an increase in resistance. Furthermore, the surface potential of the chemisorbed film, determined by the photoelectric method, was found to be positive in... 

The electronic interaction between benzene and the metal surface may be made up of two effects the polarization of the molecule, which may be concluded from the above-described research, and the shifting of the v electrons to the metal surface to become part of the metal electron gas, which has been hypothesized by Polanyi (77). The first effect has been shown in Fig. 28, the second apparently can be seen from the research (18) illustrated by Fig. 29, in which the change of resistance of a transparent nickel film was studied during the adsorption of benzene molecules. As the temperature of the benzene capsule was 90°K., the evaporation velocity was so low that only a small number of benzene molecules struck the surface in unit time. The resistance therefore diminished only... 

Change of resistance Special sensor3) 0- 8 Max. 0.3 % of span < 0.5% Proved standard method, Types of devices PMP731, PMD235 3) [54], type HP-1 [581... 

The change of resistance of a hot wire when struck by a liquid drop has been investigated 20) ... 

The coefficients for Pt are A = 4 x 10 3, B = 5.8 x 10 7, and po = 1 x 10-5 Q cm. With these parameters, the sensitivity, expressed as the temperature coefficient, is 0.4%°C 1 over a wide range of temperatures. Resistivities of other metals, as well as their temperature coefficients, are tabulated in standard reference tables (e.g., the CRC Handbook of Chemistry and Physics, 2006). Because the geometry of the resistor does not change with temperature, (3.8) is often written in terms of change of resistance R. Because of their chemical inertness and high temperature coefficient, platinum resistors are most common. They are the key part of the most successful thermal sensors, pellistors, which are discussed in Section 3.6.2. 

The relative change of resistance is its sensitivity characterized as the thermistor coefficient a. It is obtained by dividing (3.10) by (3.11). 

The band bending is macroscopically observable in a change of resistance in case of the n-type semiconductor Sn02 the resistance increases in comparison to the situation without ambient oxygen. 

Due to the proposed elementary steps of this sensing principle and the signal transduction from the surface reaction to a change of band structure and consequentially, to a change of resistance, as described in detail in (Weimar, 2002), the dependency of the sensor resistance from the concentration of analyte is logarithmic and the resistance change is dependent on the baseline value. In order to have a more transferable parameter for the characterization of sensor performance, the sensor signal S is defined as ... 

Figure 32 also shows the variation of the ballast resistance with position. The change of resistance across the pipe is much less than that of the sensor voltage. This difference demonstrates the response of the sensor electrodes to a smaller spatial region than the field electrodes. 

At moisture contents above 24 or 25%, readings are less reliable than readings below 24 or 25% for two reasons. First, the rate of change of resistance with moisture content decreases markedly, so the sensitivity is reduced. Second, the moisture content reading decreases substantially with time because of polarization effects. The latter effect can be minimized by the use of alternating current (AC) rather than the direct current (DC) instruments traditionally used for resistance meters. 

The most interesting aspect of conduction by electrons, namely superconduction, in which experiment is continually providing us with fresh surprises, scarcely falls within the scope of this lecture, as so far there exists no theory of superconduction to test. Such a theory would have to explain the characteristic conditions under which superconduction occurs, the change in the order of magnitude of the resistance, and the temperature at which the change of resistance takes place. Even empirical relationships for these have still to be discovered. 

This simple way of looking at the matter immediately leads to the result that no change of resistance should arise if the magnetic field is parallel to the direction of the current, for the component of motion parallel to the magnetic field is not modified by the field. On the contrary, however, experiments show that though the change of resistance in this case is noticeably smaller than in the case of a transverse field, it still remains of the same order of magnitude.. ... 

As might be expected, Sommerfeld s theory also makes the longitudinal effect vanish. Thus there are two contradictions between theory and experiment. A third difficulty arose when Kapitza and Meissner and Scheffers f made experiments with strong magnetic fields at low temperatures and showed that even at intensities which are quite easily attained the effect ceases to be proportional to the square of the intensity. In the strongest fields hitherto obtained the change of resistance is a linear function of the intensity. 

This evaluation says nothing about the detailed mechanism of the change of resistance the fact remains that the effects of the magnetic field taken into account by Sommerfeld give far too small a result. We must therefore consider the points which are neglected in Sommerfeld s discussion of the conductivity. 

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