Question

Derive the expression for unknown resistance using metre bridge. What will be the effect of the null point if another metre bridge wire having, a similar cross section area is used ?

Answer 1

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Explanation:

The metre bridge, also known as the slide wire bridge consists of a one metre long wire of uniform cross sectional area, fixed on a wooden block. A scale is attached to the block. Two gaps are formed on it by using thick metal strips in order to make the Wheat stone’s bridge. The terminal B between the gaps is used to connect galvanometer and jockey.

A resistance wire is introduced in gap S and the resistance box is in gap R. One end of the galvanometer is connected to terminal D and its other end is connected to a jockey. As the jockey slides over the wire AC, it shows zero deflection at the balancing point (null point).

Here, according to the wheatstone bridge principle,

x / R = ρ ( 1/A )  ÷ ρ ( 100-1 / A )

⇒ x = ( 1 / 100 - 1 ) R

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