Introduction:
In AC bridges, balance detectors are used to determine when the bridge is in a balanced condition, which means that the unknown impedance is equal to the known impedance. This is achieved by measuring the null point, where there is no voltage or current flowing through the detector.

Commonly used balance detectors:
The commonly used balance detectors for AC bridges are as follows:

1. Headphones:
Headphones are one of the simplest and most commonly used balance detectors in AC bridges. They consist of a pair of earphones connected to the bridge circuit. When the bridge is balanced, the null point is reached, and no current flows through the headphones, resulting in silence. If there is any imbalance, a small current flows through the headphones, producing an audible sound. This allows the user to easily detect when the bridge is balanced or not.

2. Tuned amplifiers:
Tuned amplifiers are another commonly used balance detector in AC bridges. They consist of an amplifier connected to the bridge circuit. The amplifier is tuned to a specific frequency, which matches the frequency of the AC signal used in the bridge. When the bridge is balanced, the null point is reached, and no signal is amplified by the tuned amplifier. However, if there is any imbalance, a signal is amplified by the tuned amplifier, indicating that the bridge is not balanced.

3. Vibration galvanometers:
Vibration galvanometers are sensitive instruments used as balance detectors in AC bridges. They consist of a coil suspended between the poles of a permanent magnet. When an AC current flows through the coil, it experiences a force due to the magnetic field, causing it to vibrate. When the bridge is balanced, the null point is reached, and no current flows through the coil, resulting in no vibration. However, if there is any imbalance, a current flows through the coil, causing it to vibrate. This vibration can be observed visually or by using a suitable detection mechanism.

Conclusion:
In conclusion, headphones, tuned amplifiers, and vibration galvanometers are commonly used balance detectors in AC bridges. These detectors allow users to easily determine when the bridge is in a balanced condition, which is important for accurate measurement of unknown impedances.