Introduction:
The wattmeter is an instrument used to measure electrical power in AC circuits. It can measure both the active and reactive power components. There are different types of wattmeters available, but the most commonly used type for AC power measurement is the dynamometer type wattmeter.

Dynamometer Type Wattmeter:
The dynamometer type wattmeter is based on the principle of electromagnetic induction. It consists of two coils - a fixed coil called the current coil and a movable coil called the potential coil. The current coil is connected in series with the load, and the potential coil is connected across the load. The current coil produces a magnetic field proportional to the current flowing through it, while the potential coil produces a magnetic field proportional to the voltage across it.

Operating Principle:
When an AC current flows through the current coil, it produces a magnetic field that interacts with the magnetic field produced by the potential coil. The interaction of these magnetic fields creates a torque on the potential coil, causing it to rotate. The angle of rotation of the potential coil is directly proportional to the power being measured.

Measurement of Power:
The power can be measured by determining the torque experienced by the potential coil. This torque is directly proportional to the power being measured. A calibrated spring is used to counterbalance the torque, allowing the measurement of power.

Advantages of Dynamometer Type Wattmeter:
1. Accuracy: The dynamometer type wattmeter provides accurate measurements of power in AC circuits.
2. Wide Frequency Range: It can measure power across a wide range of frequencies.
3. High Overload Capacity: It can handle high overload currents without getting damaged.
4. Low Power Consumption: The power consumption of the wattmeter itself is relatively low.

Conclusion:
In conclusion, the dynamometer type wattmeter is commonly used for the measurement of power in AC circuits. It is based on the principle of electromagnetic induction and provides accurate measurements of power. Its advantages include accuracy, wide frequency range, high overload capacity, and low power consumption.