Draw the Diagram of Meter Bridge
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A resistance box R and an unknown resistance S is connected across the two gaps of the metallic strips as shown in the figure. One end of the galvanometer is connected to the middle lead of the metallic strip which is placed between the L shaped strips. The other end of the galvanometer is connected to a jockey. The jockey is used to slide on the bridge wire. It is a metal rod with one end as knife edge.
Meter Bridge
Now adjust the value of resistance in the resistance box and slide the jockey along the wire. This process is to be done until the galvanometer shows a zero or null deflection. Consider at point B the galvanometer showed zero deflection. So now the jockey is connected to the point B on the wire. Thus the distance from the point A to B is taken as L1 cm. Then the distance from point B to point C is L2 cm that is 100 - L1 cm. Now the meter bridge becomes similar to the Wheatstone bridge. The meter bridge now is drawn as Wheatstone bridge for more clearance.
Meter bridge to Wheatstone bridge
We know that R = ρ L /A, where R is the resistance, ρ is the resistivity, L is the length of the wire and A is the area of cross section of the wire. So the resistance across the portion AB of the wire is Rw L1 and that the resistance across the point BC is Rw (100 - L1). Rw is the resistance of the whole wire. That is according to Wheatstone bridge we get
This is the equation
So to find the unknown resistance S we have S = R (100 - L1) / L1
Meter Bridge
Now adjust the value of resistance in the resistance box and slide the jockey along the wire. This process is to be done until the galvanometer shows a zero or null deflection. Consider at point B the galvanometer showed zero deflection. So now the jockey is connected to the point B on the wire. Thus the distance from the point A to B is taken as L1 cm. Then the distance from point B to point C is L2 cm that is 100 - L1 cm. Now the meter bridge becomes similar to the Wheatstone bridge. The meter bridge now is drawn as Wheatstone bridge for more clearance.
Meter bridge to Wheatstone bridge
We know that R = ρ L /A, where R is the resistance, ρ is the resistivity, L is the length of the wire and A is the area of cross section of the wire. So the resistance across the portion AB of the wire is Rw L1 and that the resistance across the point BC is Rw (100 - L1). Rw is the resistance of the whole wire. That is according to Wheatstone bridge we get
This is the equation
So to find the unknown resistance S we have S = R (100 - L1) / L1
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