Why does a wattmeter have two coils?

Short Answer:

A wattmeter has two coils—a current coil and a voltage (pressure) coil—because it needs to measure both current and voltage at the same time to calculate power. The current coil is connected in series with the load to measure current, while the voltage coil is connected in parallel across the load to measure voltage.

The interaction between the magnetic fields produced by these two coils creates a torque that moves the pointer. This torque is proportional to the real power (P = VIcosφ), which the wattmeter displays as a reading in watts. Both coils are essential for accurate power measurement.

Detailed Explanation:

Reason for two coils in a wattmeter

A wattmeter is a device used to measure electrical power in a circuit. The electrical power depends on both voltage and current, and in AC circuits, it also depends on the power factor. To measure power accurately, a wattmeter must detect both current and voltage at the same time. That is why a wattmeter is designed with two separate coils—one to sense current and the other to sense voltage.

  1. Current Coil (Series Coil):
  • This coil is connected in series with the load.
  • It carries the full load current of the circuit.
  • It creates a magnetic field proportional to the current flowing through the circuit.
  • Being in series, it experiences the same current as the circuit load.
  1. Voltage Coil (Pressure Coil):
  • This coil is connected in parallel across the load.
  • It carries a small current due to the high resistance in series with it.
  • It produces a magnetic field proportional to the voltage applied.
  • The current in this coil is proportional to the voltage being measured.

Interaction Between the Two Coils:

The wattmeter functions based on the electromagnetic interaction between the magnetic fields produced by these two coils. When both coils are energized:

  • The fixed coil (current coil) generates a magnetic field due to current.
  • The moving coil (voltage coil) is placed within this field and carries a current proportional to the voltage.
  • The interaction between these two fields generates a deflecting torque on the moving coil.
  • This torque is proportional to the product of current, voltage, and the cosine of the phase angle (cosφ) in AC circuits.

Td∝V×I×cos⁡ϕT_d \propto V \times I \times \cos\phiTd​∝V×I×cosϕ

This torque moves the pointer on the scale, indicating the real power consumed in the circuit.

Why Two Coils Are Necessary:

  • Power is not a single quantity like voltage or current—it depends on both.
  • One coil alone can only measure either current or voltage, not both.
  • Using two separate coils allows the meter to combine both measurements into one reading of true power.
  • Especially in AC circuits, the phase difference between voltage and current must be considered, which is only possible with the interaction of both coils.

Applications:

  • Accurate power measurement in AC and DC circuits.
  • Used in power testing of appliances and industrial equipment.
  • Suitable for laboratory experiments requiring precise power readings.
  • Helps calculate power factor and energy consumption when combined with other instruments.
Conclusion

A wattmeter has two coils because it must measure both current and voltage simultaneously to calculate true electrical power. The current coil handles the load current, and the voltage coil senses the potential difference. Their combined magnetic interaction produces a torque that directly corresponds to power. Without both coils, the wattmeter cannot function properly or deliver accurate results.