# Wheatstone Bridge-

Before you go through this article, make sure that you have gone through the previous article on Wheatstone Bridge.

We have learnt-

• A Wheatstone bridge is an arrangement of four resistances used to determine one of these resistances in terms of the remaining three resistances.
• It is a null method and is preferred over the other methods for determining the value of an unknown resistance.

# Meter Bridge-

 Meter bridge is the practical application of the Wheatstone bridge that is used to measure an unknown resistance.

## Principle-

Its working is based on the principle of Wheatstone bridge. When the bridge is balanced, we have-

## Construction-

• It consists of a one meter long manganin wire of uniform cross-section.
• The wire is stretched along a meter scale fixed over a wooden board.
• The two ends of the wire are soldered to two L-shaped thick copper strips A and C.
• Between these two copper strips, another copper strip is fixed so as to provide two gaps.
• A resistance box is connected in one gap and the unknown resistance in the other gap.
• A source of emf E is connected across AC.
• A movable jockey and a galvanometer are connected across BD as shown in the figure.

## Working-

• A suitable resistance R is taken out from the resistance box.
• Then, the jockey is moved along the wire AC till there is no deflection in the galvanometer.
• This is the balanced condition of the Wheatstone bridge.

If P and Q are the resistances of the parts AB and BC of the wire, then for the balanced condition of the Wheatstone bridge, we have-

Let AB = ℓ cm, then BC = (100 – ℓ) cm.

Since the wire is of uniform cross-section, therefore-

Resistance of wire ∝ Length of wire

where σ is the resistance per unit length of the wire.

Hence,

From here,

Knowing the values of ℓ and R, the value of unknown resistance S can be determined.

## Determination of Resistivity-

If r is the radius of the wire and L is its length, then resistivity of its material is given by-

Quiz on Meter Bridge

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Electric Cells & Batteries

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