State the principal and working of a meter bridge?

Principle: Meter bridge is based on the principle of wheatstone bridge.

i.e. when bridge is balanced,

Where, R1 and R3 = known resistances,

R2 = Variable resistance and

R4 = unknown resistance


In this bridge one lateral pair of resistances is replaced by a straight wire of length 1m of uniform cross section. And other pair consist of one known and one unknown resistance. One side of a galvanometer is connected between known ad unknown resistance and other side is moved along the wire to find the null point i.e. at which galvanometer doesn't show any deflection. This point balances the bridge.

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we know that, R = rho*L/A ,R=resistance, rho= specific resistance, L=length of conductor, A=area of cross-section

here, if specific resistance(rho) & area of cross-section(A) becomes constant then 

         R will be directly proportional to L(length),

hence the restance(R) can be found with the help of the length of the conductor(L)

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