Resistance measurement Wheatstone bridge

Here, the unknown resistance is compared with a known resistance using a suitable bridge. Resistance above I Q. can be measured by Wheatstone bridge. Resistance less than I Q can be measured by a Kelvin double bridge, where the lead resistance must also be compensated. 

In its simplest form, the Wheatstone bridge is used on D.C. for the measurement of an unknown resistance in terms of three known resistors. Its accuracy depends on that of the known units and the sensitivity of the detector. It is also used for sensing the changes which occur in the output from resistance strain-gauge detectors. The latter instruments can be made portable and can detect variations of less than 0.05 per cent. 

Electrical resistance monitors use the fact that the resistance of a conductor varies inversely as its cross-sectional area. In principle, then, a wire or strip of the metal of interest is exposed to the corrodent and its resistance is measured at regular intervals. In practice, since the resistance also varies with temperature, the resistance of the exposed element is compared in a Wheatstone bridge circuit to that of a similar element which is protected from the corrodent but which experiences the same temperature. 

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. 

The accurate measurement of resistance can of course be carried out by means of a Wheatstone bridge, Carey Foster bridge, or a similar arrangement. 

Fig. 21 The variation of the balancing tunneling current of the four branches four electrodes monomolecular Wheatstone bridge connected as presented in (a). In (b), the dashed line is for the current intensity 7W (in absolute value) measured by the ammeter A and deduced from the standard Kirchoff laws calculating each molecular wire tunneling junction resistance of the bridge one after the other from the EHMO-ESQC technique. In (b), Hie full line is the same tunnel current intensity but obtained with the new intramolecular circuit rules discussed in Sect. 2. (c) The resistance of the branch used to balance the bridge as a function of its rotation angle. The minimum accessible resistance by rotation is 78 MQ for the short tolane molecular wire used here...

The platinum resistance may have the form of a coil supported on a crossed mica web or of a bird-cage element, or of a bifilar wire wound on a ceramic bobbin or of a film deposited on a small ceramic substrate. The resistance can be measured using a bridge (Wheatstone bridge) or a four-wire ohmmeter (Hewlett Packard 1997). For curve fitting of the data the following equation is frequently used ... 

In a null comparison measurement of resistance, the effect of an unknown resistance must be compared with the effect of a variable standard resistance under conditions as identical as possible. Therefore, the unknown and standard resistances are placed in identical circuits in such a way that the resulting voltage or current in each circuit can be compared. Then the standard is varied until the difference in voltage or current between the two circuits is zero. Several methods for performing this comparison have been devised, of which the Wheatstone bridge is by far the most common. Comparison methods for resis-... 

The Wheatstone bridge shown in Figure 8.5 provides the most direct and best known circuit for comparison of unknown resistances against standard resistances. Resistances RA, RB, and R are standard resistance values that are used in the measurement of the unknown resistance R Resistance R is made variable and is adjusted until the null detector indicates that the bridge is balanced. 

Traditionally, the instrument of choice for accurate conductance measurements that are relatively free of capacitance effects has been the ac Wheatstone bridge illustrated in Figure 8.14. The details of operation and the derivation of the balance condition of the ac bridge are presented in considerable detail elsewhere [16,17], The balance condition is exactly analogous to that of the dc bridge except that impedance vectors must be substituted for resistances in the arms of the bridge when reactive circuit elements are present. 

Strain Gages. Essentially, they are based on electrical measurements of strains produced by the action of stress. There are several ways for measuring strains, but the most convenient is the resistance method. The apparatus used for these measurements, known as "resistance manometer , depends upon the change in resistance of a metal (such as platinum, manganin or mercury) when subjected to pressure. The change in resistance is usually measured by means of a very sensitive Wheatstone bridge, but it may also be measured by a potentiometer or by an... 

These measure the change in thermal conductivity of a gas due to variations in pressure—usually in the range 0.75 torr (100 N/m2) to 7.5 x 10"4 torr (0.1 N/m2). At low pressures the relation between pressure and thermal conductivity of a gas is linear and can be predicted from the kinetic theory of gases. A coiled wire filament is heated by a current and forms one arm of a Wheatstone bridge network (Fig. 6.21). Any increase in vacuum will reduce the conduction of heat away from the filament and thus the temperature of the filament will rise so altering its electrical resistance. Temperature variations in the filament are monitored by means of a thermocouple placed at the centre of the coil. A similar filament which is maintained at standard conditions is inserted in another arm of the bridge as a reference. This type of sensor is often termed a Pirani gauge. 

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