Question

Explain meter bridge

Answer 1

The meter bridge is an instrument used to measure an unknown electrical resistance based on the Wheatstone bridge principle. It consists of a uniform wire, known and unknown resistances, and a galvanometer to find the null point, allowing calculation of the unknown resistance.

Answer 2

Explanation of Meter Bridge

The meter bridge is an instrument used to measure an unknown electrical resistance. It is based on the principle of the Wheatstone bridge.

Principle:

The meter bridge works on the principle of a balanced Wheatstone bridge. A Wheatstone bridge is balanced when the ratio of resistances in one pair of arms is equal to the ratio of resistances in the other pair of arms, resulting in zero current flowing through the galvanometer connected across the bridge.

Construction:

A meter bridge consists of:

1. A 1-meter long wire of uniform cross-section and made of a material with high resistivity (like manganin or constantan), stretched along a scale marked 0 to 100 cm.

2. Two L-shaped thick copper strips and one straight thick copper strip, which provide low resistance connections.

3. Two gaps, G1 and G2, formed between the copper strips. These gaps are used to connect the unknown resistance (S) and a known resistance (R) from a resistance box.

4. A battery (or DC source) connected across the ends of the meter wire (points A and C).

5. A galvanometer (G) connected between the central copper strip (point B) and a jockey (J). The jockey is a sliding contact that can be moved along the meter wire.

Here is a schematic diagram:

A and C are the ends of the meter wire. R is the known resistance, S is the unknown resistance. G is the galvanometer, J is the jockey.

Working:

1. Connect the unknown resistance S in gap G2 and a known resistance R from a resistance box in gap G1.

2. Connect the battery across points A and C.

3. Connect the galvanometer between point B and the jockey J.

4. Slide the jockey along the meter wire until the galvanometer shows zero deflection. This point is called the null point (J).

5. Let the distance of the null point from end A be l cm. Then the distance of the null point from end C will be (100 - l) cm.

6. At the null point, the Wheatstone bridge is balanced. The four resistances forming the bridge are R, S, the resistance of the wire segment AJ (length l), and the resistance of the wire segment JC (length 100 - l).

7. Let r be the resistance per unit length of the meter wire. The resistance of segment AJ is R_AJ = r * l and the resistance of segment JC is R_JC = r * (100 - l).

8. According to the Wheatstone bridge principle for a balanced bridge: $\frac{R}{S} = \frac{R_{AJ}}{R_{JC}}$ $\frac{R}{S} = \frac{r \times l}{r \times (100 - l)}$ $\frac{R}{S} = \frac{l}{100 - l}$

9. From this equation, the unknown resistance S can be calculated: $S = R \times \frac{100 - l}{l}$

By knowing the value of R and measuring the null point length l, the unknown resistance S can be determined.