Explain in detail about construction woking of single phase wattmeter?

Wattmeter

Definition

The wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit.Electric power is measured by means of a wattmeter. This instrument is of the electrodynamic type. It consists of a pair of fixed coils, known as current coils, and a movable coil known as the potential coil. The fixed coils are made up of a few turns of a comparatively large conductor. The potential coil consists of many turns of fine wire. It is mounted on a shaft, carried in jeweled bearings, so that it may turn inside the stationary coils. The movable coil carries a needle which moves over a suitably marked scale. Spiral coil springs hold the needle to a zero position.The figure below shows the circuit of a simple wattmeter.