Experiment: Determine the specific resistance of the material of two given wires using a metre bridge.
Metre bridge is the practical form of Wheatstone’s bridge where
P/Q = R/S
P and Q are called ratio arms, R is adjustable and S is the unknown resistance. For a wire of uniform area of cross-section, if null point obtained at length l,
P/Q = σ/(100-l)σ = 1/(100-l)
As the total length of the wire of metre bridge is 100 cm, where σ is resistance per unit length of the bridge wire. Therefore,
S = (100-l)/l × R
A metre bridge, a galvanometer, a jockey, a Leclanche cell, a one way key, a resistance box, a metre scale, sandpaper, connecting wires and screw gauge.
1. Draw the circuit diagram and make the connections according to the circuit diagram.
2. Remove the insulations from the ends of the connecting wires with the help of sand paper and make neat, clean and tight connections.
3. Make sure that the resistance in the resistance box is of same order of magnitude as the unknown resistance S.
4. To check whether the connections of the circuit are correct, take out a plug from the resistance box to introduce suitable resistance in the circuit. Open the key K. Now the 1000 Ω resistor makes galvanometer safe. Touch the jockey gently, first at the left and then at the right end of the metre bridge wire. If the deflections in the galvanometer are in opposite directions, the connections are correct.
5. Now choose an appropriate resistance R from the resistance box. This is the rough position of null point. Now close the key K and then make fine adjustment of null point. Slide the jockey on the metre bridge wire gently by touching and lifting it again and again till the galvanometer reads zero nearly in the middle of the wire.
6. Record the lengths of both parts of the wire in the observation table.
7. Repeat the above steps two times more by selecting the suitable values of R for getting null point between 30 cm and 70 cm.
8. Now cut the resistance wire S at the points where it leaves binding terminals. Straighten it by stretching and remove 3 kinks.
9. Measure the diameter of the wire by a screw guage at least the different points. At each point, the diameter should be measured in two mutually perpendicular directions.
10. Repeat the whole experiment for second wire of different mater.