Metrebridge

 Application of Wheatstone Bridge

A) Metrebridge : It is a device used to measure unknown resistance based on wheatstone network.The length of the wire is 1m, therefore it is called as metrebridge.

Construction : It consist of a uniform conductor of 1 metre mounted on a wooden board as shown in diagram below. A unknown resistance X is connected on one side of the bridge and a resistance box having known value R is connected on other side of the bridge.A galvanometer along with jockey is mounted between two resistance to form a wheatstone network.

Diagram: 

Working : when the potential difference is applied between two ends of the conductor having length 1 metre, the current is divided at junction according to kirchoff's current law. The jockey of the galvanometer is tapped on the wire in order to adjust both end of the galvanometer at same potential so that the current through galvanometer become zero. Let us consider point L₁ is obtained from left such that at this point galvanometer shows null deflection. The value of length is noted and L₂ is obtained using mathematical calculation. Now bridge is said to be in balanced condition.

According to wheatstone bridge,

X / R = (ρL₁ / A) / (ρL₂ / A) ----------- ( R= (ρL / A)

X / R = L₁ / L₂

X / R = L₁ / (100- L₁) --------------(A)   ( L₂ = 100 - L₁)

Equation no (A) gives value of unknown resistance .

Errors in Experiment: 

1)  If the cross section of the wire is not uniform , it will leads to error in measurement.

2) The end where the wire are joined , a contact resistance is developed.

3) There might be problem in measuring length.

Precautions to reduce errors:

1) The resistance should be so adjusted that null deflection is  obtained between 33cm to 66cm.

2) Take minimum 3 reading and obtain mean.

3) The jockey should be tapped on the wire and it should not be slided on it.

4) try experiment by interchanging the position of unknown resistance and resitance box.

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