WHEATSTONE BRIDGE

Charles Wheatstone.
However, Wheatstone is best known for his contributions in the development of the Wheatstone bridge, originally invented by Samuel Hunter Christie, which is used to measure an unknown electrical resistance, and as a major figure in the development of telegraphy.

A Very important device in the measurement of medium resistances is the Wheatstone. 

A Wheatstone bridge has been in use longer than almost any electrical measuring instrument. ti is still an accurate and reliable instrument and is extensively in industry.

The Wheatstone bridge is an instrument for making comparison measurements and operates upon null indication principle.

This means the indication is independent of calibration of the null indicating instrument or any of its degrees accuracy can be achieved  using Wheatstone bridge.Accuracy of 0.1% is quite common with a Wheatstone bridge as opposed ordinary ohmmeter.

Fig. Shows the basic circuit of wheatstone brige . it has four resistance arms, consisting of resistances P,Q, R and S together with source of emf and null detector , Usually a Galvanometer G .

At Balance Condition:

The Bridge said to be balanced when There is  no current flow though the Galvanometer or when potential difference across the galvanometer is zero.

This occurs when voltage From b to a is equal to voltage from d to a or by referring to the other battery terminal ,  when voltage From d to c is equal to voltage from b to c.

Bridge Balance

Where R is the unknown Resistance S is called the Standard arm of the bridge and P and Q are called the ratio arms.

At UnBalance Condition:

When the current though the Galvanometer is called unbalance condition. the galvanometer current can be found out by finding the thevenin equivalent circuit. the thevenin or open circuit voltage appearing  terminal b and d with galvanometer circuit open circuit is ,

Measurement Error:

1. Insufficient sensitivity of galvanometer.
2. Change in resistance of the bridge arms due to the heating effect of current though of current though the resister .
3. Thermal emf in the bridge circuit or galvanometer circuit can also causes problem when low value resister are being measured. 
4. Error due to load resistance and contacts to the actual bridge circuit play a role in very low resistance measurement.