What Is the Principle Behind the Analog Single Phase Wattmeter?

An analog wattmeter works on the same principle as an electrodynamometer, namely that when the same current passes through two concentric coils at right angles to each other, the torque produces depends on the square of the current. Though Wilhelm Weber introduced the basic principle in 1848, it was not until 1880 that Werner von Siemens used it to build an electrodynamometer.

Analog wattmeters are similar to early electrodynamometers because they have a pair of fixed coils called current coils. They also include a movable coil. This movable, or potential, coil is connected in parallel with the circuit, and carries a needle that indicates current measurement as it moves over a scale. The two current coils are connected in series to the circuit, but can also be connected in parallel to each other to change the range of the wattmeter. Analog wattmeters measure true power when connected to an AC circuit.