Page 16 - Physics - XII

P R
Thus, = …(1)
Q S
Since, the resistance (R) of a conductor is directly proportion to the length (l) of the conductor.

i.e. R ∝ l

Thus for metre bridge wire,
Resistance ratio = Length ratio

In the metre bridge, at null defl ection in the galvanometer or when the bridge gets balanced,

P = R = Length AD of bridge wire
Q S Length DC of bridge wire

∴ X R l

Q S 100 l

l
∴ Unknown resistance, X Q
100 l

Q is the value of known Resistance from resistance box.

Procedure

1. Clean the insulation at the ends of the connecting wires with sand paper. Tighten all the plugs of the
resistance box (RB) by pressing and twisting each plug.

2. Connect the wire whose resistance is to be determined in the left gap E of the metre bridge.
3. Introduce a resistance Q from the resistance box in the circuit.

4. Insert the plug into the one-way key K.

5. Now, touch the jockey gently at the left end and then at right end of the metre bridge wire and note
the direction of defl ection in the galvanometer. If the galvanometer shows defl ections in opposite
directions at both ends then, the connections are correct.

6. Now, move the jockey gently along the wire from left to right till galvanometer gives no defl ection.
This point where the jockey is touching the wire is null point D, and at D the bridge is in balancing
position.

7. Choose an appropriate value of Q from the resistance box such that there is no defl ection in the
galvanometer when the jockey is nearly in the middle of the wire, i.e. between 30 cm to 70 cm.

8. Note the position of point D and record the length AD = l.
9. Take at least four such observations by changing the value of Q in the step of 1 Ω.

10. Note all the observations in observation table.

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