METER BRIDGE EXPERIMENT

METER BRIDGE

AIM:To determine the unknown resistance of a given wire using a metre bridge.

APPARATUS:Leclanche cell,a center zero galvanometer, resistance box, sliding contact or Jockey meter bridge.

DIAGRAM:

THEORY:The meter bridge operates under the wheatstone principle.here the current from the battery divides between the two branches ABC and ADC.since the potential drop is the same along the two branches, corresponding intermediate points B and D may be found which are at the same potential.under these circumstances no current will flow through a galvanometer connected between B and D.the bridge is then said to be balanced and then R1/R2=R3/R4.

METER BRIDGE:This is one practical arrangement of the wheatstone bridg.in this form,one of the branches ADC consists of a wire of uniform cross section and of length 100cm.the point D is located by a sliding contact.the unknown resistance X is placed in one arm of the other branch,the remaining arm containing the known resistance R which usually is in the form of a resistance box as shown below.

   it will be seen that the resistance of the portions AD and DC of the wire replace the R3 and R4 of figure 20a,hence the balance conditions is given by.

  X/R=resistance of AD/resistance of DC

But since the wire is uniform,it's resistance is directly proportional to it's length.

   Thus,

X/R=L/(100-L)

PROCEDURE

:The circuit was set up as indicated in the above diagram in which X is the unknown resistance,and R is a standard resistance.before switching on the current the following were observed carefully.

1.The uniformity of the wire is very important, hence it was ensured that the contact between the Jockey and the wire was light.

2.Touch the Jockey at the point A,it was noticed that there was a deflection in the galvanometer on one of the sides.then contact was made on the Jockey at point C of the wire,the above procedure was used for both the think and thick wire used separately as the unknown resistance.

4. To 6.

CONCLUSION

the unknown resistance X for the thin anf thick wires respectively are 0.48 and 3.05 and the specific resistance(resistivity) for the thin wire is greater than that of the thick.

PRECAUTION

1. It was ensured that the wire use was uniform, the jockey was touched lightly by the wire.

2. It was ensured that before seeking the null point, the key was closed before contact was made at D to avoid deflection due to the inductor effect.

3. It was ensured that the small current was passed through the galvanometer A to avoid damage.