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WHAT ARE THE PARTS OF A METER BRIDGE?HOW IS ITS OPERATION?

Metre Bridge (Slide Wire Bridge)

Principle: It is based on the principle of balanced Wheatstone bridge.

Construction : It consists of uniform resistance wire AC, usually one metre long, soldered to the ends of the two thick rectangular strips of copper AE and CF, fixed on a wooden base. A sliding contact B, called the jockey, can be moved along a graduated scale, if the length of the wire is 1m, it is called metre bridge. DH is another copper strip placed between H and F. These gaps are attached with two resistors, one of known value and other unknown. A battery E is connected between A and C with a key K in series as shown in the below figure.

Working: The unknown resistance X is placed in the gap HF and a known resistance R is placed across the gap ED. The two resistance P and Q are obtained from the wire AC by moving the jockey B connected to the strip D through a galvanometer G.

                             The key K in the circuit is inserted and the jockey is moved along the wire till, for a certain position B, the galvanometer shows no deflection. Th bridge is then said to be balanced. If P and Q are the resistances of the parts AB and BC of the wire we have,

If the slide wire is of uniform cross-section, the resistance of AB and BC shall be proportional to their lengths say l and 100-l respectively. If ? is the resistance per unit length of the wire, we have  

Substituting in the above equation, we have

Knowing l and R, X can be calculated.