The general arrangement of Wheatstone bridge is shown in. It consists of four arms AB, BC, CD and DA consisting of known and fixed resistances P and Q, known and variable resistance S and unknown resistance R respectively. Arms AB and BC having resistances P and Q respectively are known as ratio arms. A sensitive galvanometer G is connected through switch S₂ across the terminals B and D. The battery is connected through the switch S₁ across other two terminals A and C. The galvanometer G s protected by providing a shunt N which avoid, flow of heavy current and large deflection when the bridge is unbalanced. The resistance of shunt N is increased step by step as the bridge reaches the balance point so that more and more current may flow through the galvanometer and increases its sensitivity.

While balancing the bridge, the switch S1 is closed, the values of resistances P,Q and S are so adjusted that the deflection in galvanometer is nil when the switch S2 is closed.

With the bridge balanced the current flowing through resistance P also flows through resistance Q. the current flowing through resistance R also flows through resistance S and the points B and D are at the same potential i.e. voltage drop from point A to point D and voltage drop from point to point C muxt be equal to voltage drop from point D to point C.

Hence I₁ P = I₂ R
          I₁ P = I₂ S

Dividing expression (i) by expression (ii) we get
              
           P  = R
         Q     S

or       R = P  X S
               Q

The resistance P and Q of the bridge are made of definite ratio such 1:1:10:1 or 100:1 known as ratio arms and S the arms and S the through are is made continuously variable form 1 to 1,000 Ω or form 1 to 10,000 Ω .

For more help in Wheatstone Bridge click the button below to submit your homework assignment