Question

Write the method to determine the value of an unknown resistance by meter bridge and derive necessary formula. Draw a circuit diagram.

Answer 1

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,

$\frac{X}{R}=\frac{\rho \left(\frac{l}{A}\right)}{\rho \left(\frac{100-l}{A}\right)}$

$⟹X=\left(\frac{l}{100-l}\right)R$