Answer:

A Wheatstone bridge is a circuit that is used to measure unknown resistances by comparing them with known resistances. It consists of four resistors connected in a diamond shape with a voltage source connected across the top and bottom points of the diamond. The unknown resistance is connected between the two middle points of the diamond.

The null detector is an instrument used to determine when the bridge is balanced, meaning that there is no current flowing through the galvanometer or voltmeter connected in the circuit. At balance, the ratio of the two known resistances is equal to the ratio of the two unknown resistances.

In a Wheatstone bridge, a galvanometer is used as the null detector. A galvanometer is an instrument that detects and measures small electric currents. It consists of a coil of wire suspended in a magnetic field, which causes the coil to rotate when a current flows through it.

Reasons why a Galvanometer is used as a Null Detector:

1. Sensitivity: Galvanometers are highly sensitive instruments, capable of detecting even the smallest currents. This sensitivity is important in a Wheatstone bridge, where the goal is to balance the circuit and achieve zero current through the null detector.

2. Zero-center scale: Galvanometers typically have a zero-center scale, which means that the needle is at rest when no current is flowing through the coil. This is ideal for a null detector because it allows for easy interpretation of the balance condition. If the needle deflects to the left or right, it indicates that the bridge is unbalanced and adjustments need to be made.

3. Low resistance: Galvanometers have a low internal resistance, which minimizes the effect of the null detector on the circuit being measured. This ensures accurate measurements and does not significantly alter the resistance values in the Wheatstone bridge.

4. Quick response time: Galvanometers have a fast response time, allowing for quick adjustments to be made to the bridge to achieve balance. This is particularly useful when fine-tuning the bridge to obtain accurate resistance measurements.

Conclusion:

In a Wheatstone bridge, a galvanometer is used as a null detector due to its sensitivity, zero-center scale, low resistance, and quick response time. These characteristics make it an ideal instrument for detecting and measuring small electric currents in order to achieve balance in the bridge and accurately measure unknown resistances.