Measurement of resistance has become one of the most important necessities in industries of modern time. And so for this purpose, many techniques have been introduced now a day. One of such method for measurement of resistance is the Wheatstone bridge method. This bridge is used for the measurement of medium resistances that normally range from 5S upwards to 100KS.
The Wheatstone bridge as mentioned above is used for the measurement of medium resistances. This bridge is used for making comparison measurements and operates on the null indication principle. i.e., at balanced condition, the meter when connected across the two arms of the bridge, other than the power supply arms, shows zero or null. This means the indication is independent of the calibration of the null indicating instrument or any of its characteristics. The circuit of a basic Wheatstone bridge is as shown below.
It has four resistance arms, consisting of resistances P, Q, R and S together with a source of emf and a null indicator which is usually a galvanometer. The current through the galvanometer depends on the potential difference between points 'c' and d’. The bridge is said to be balanced when there is no current through the galvanometer or when the potential difference across the galvanometer is zero. This occurs when the voltage from point 'b' to point 'a' equals the voltage from point’d’ to point 'b' or when the voltage from point 'd' to point 'c' equals the voltage from Point 'b' to point 'c'
Where 'R' is the unknown resistance and the other arms consist of ratio and standard arms.
The bridge sensitivity decreases considerably if the ratio P/Q = R/S is greater or smaller than unity. This reduction in sensitivity is accompanied by a reduction in accuracy with which a bridge can be balanced.
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