Meter Bridge

The meter bridge is a laboratory instrument used to measure the resistance of a metal coil (or any other substance). Because it is composed of a constantan (or manganin) wire 1 meter in length and has a uniform CSA, it is known as a meter bridge (cross-sectional area).

Meter Bridge

A meter bridge is an electrical instrument that allows us to determine the value of an unknown resistance by measuring the resistance. It is constructed from a meter-long wire with a consistent cross-section. This wire is either nichrome, manganin, or constantan in composition because they have high resistance and a low coefficient of resistance at low temperature.

Principle of Meter Bridge

The meter bridge principle is based on the Wheatstone Bridge circuit, which states that if at any point or length (of a wire), the ratio of two resistances (say R1 and R2) is equal to the ratio of another two resistances (say R3 and R4, where R4 is the unknown resistance), then there will be no current flow between those points and the edges containing the resistances (R1/R2 and R3/R4). As a result, when applied to the meter bridge, the galvanometer will indicate zero deflection at any point along its length.

Working in Meter Bridge 

1. To begin, move the jockey to the wire’s ends, letters A, B, and C, where A is the starting point of jockey on the meter wire, B is the null point, and C is the ending point on the wire. The deflection of the galvanometer should be the same on both ends and the opposite on one end.

2. Starting from side A, slowly slide the jockey along the wire, paying close attention to where the deflection of the galvanometer reaches zero.

3. If a stable point cannot be reached, try adjusting the resistance across the bridge by modifying the variable resistance.

4. Slide the jockey along the wire and carefully notice the place on the wire where the deflection of the galvanometer is zero. This is the null point presented by point B.

5. Measure the length of the null point using the meter scale that has been affixed to the wire. This is the length of the meter bridge that serves as a balancing length.

6. Assume that the distance between locations A and B equals ‘l.’

7. Assume that the distance between locations B and C is ‘la,’ where l2 =100 – l1.

Procedure for the Meter Bridge experiment

1.  Remove a suitable type of resistance from the resistance box ‘R.’
2.  Position the jockey at point A, and check to see whether there is a deflection within the galvanometer. The deflection of the galvanometer must shift from one side to the other when the jockey is moved from point A to position C. If this is not detected, the known resistance value should be adjusted.
3. Move the jockey from A to C, aiming for the position where there is no deflection of the galvanometer.
4. Continue using the previous technique for various ‘R’ values. Take note of around 5-10 readings.
5. The balancing point of the meter bridge is when the galvanometer shows no deflection for the specified unknown resistance.
6. Using an ordinary meter scale and a screw gauge, measure the distance between point A and the balance point of a specified wire and the radius of the wire (Take at least five readings for both the quantities).
7. Calculate the mean value of the unknown resistance acquired in step 7. In this case, the total resistance will be equal to the sum of all resistance values divided by the total number of readings collected.

Mistakes to avoid and measures to take when using a meter bridge

Errors that might occur: Non-uniformity in the wire results in varying resistances per unit length, resulting in an inaccuracy in the unknown resistance. Furthermore, inaccuracies owing to the heating effect and corrections introduced due to the movement of the scale’s zero may occur.

Precautions while working with a meter bridge

• Whenever possible, the battery key should be depressed before making the galvanometer contact on the bridge wire to guarantee that a continuous current flows through the circuit.
• Otherwise, a counter EMF would be self-induced somewhere in the circuit, making it difficult to determine the exact balancing point.
• The experiment should be performed with the resistances in the left and right gaps being swapped.
• The resistance of the wire is set at a value that allows the equilibrium condition to be reached near the middle of the wire.

Conclusion

A meter bridge is an electrical device that allows us to measure the value of unknown resistance. It’s fashioned from a meter-long wire with a consistent cross-section. This wire is composed of nichrome, manganin, or constantan. The working principle of a meter bridge is the same as the working principle of a Wheatstone bridge. The theory of null deflection underpins a Wheatstone meter bridge.