Wheatstone Bridge

                                           Wheatstone Bridge

Wheatstone is a device used for measuring resistance ( 10 ohm to 100 ohm approximately).It was discovered by Charles wheatstone (1807-1875) .

Principle : It works on the principle of null deflection. When the potential between two point is equal no current flow through galvanometer and the bridge is said to be in balanced condition.

Construction : It consist of four resistance connected in a manner to form a Quadrilateral ABCD. as shown in fig below. A potential difference is connected between point A and C such that point A is at highest potential and point C is at lowest potential. A galvanometer is connected between point B and D to form a bridge.

Diagram : 

Solution: When the potential difference is applied between two point A and C , the current I start flowing through the circuit .At point A the current gets divided as  I₁ in branch AB and  I₂ in branch AD. The galvanometer is connected such that the potential at point B and D are at same potential so that no current flow through the galvanometer i.e Ig = 0. (Balanced condition is achieved)

Applying KVL in loop ABDA,

I₁ R₁ + 0  - I₂ R₃ = 0

I₁ R₁ = I₂ R₃

I₁ / I₂ = R₃ / R₁ -----------(A)

Applying KVL in Loop BCDB,

(I₁-Ig)R₂ -(I₂+Ig)R₄ - Ig =0

putting Ig =0 as no current flow through galvanometer

I₁ R₂ = I₂ R₄

I₁ / I₂ = R₄ / R₂ -----------(B)

 From A and B,

 

R₃ / R₁ = R₄ / R₂

adjusting above equation,

R₁ / R₂ = R₃ / R₄ ------------(3)

With the help of equation (3) we can find out unknown resistance if three resistance is known.

                                   <<< KVL & KCL│ │ Metre's Bridge>>> 

---