Dynamometer type Wattmeter:

Principle:  When a current carrying moving coil is placed in a magnetic field produced by the current carrying fixed coil, a mechanical force is exerted on the coil sides of the moving coil and deflection  takes place.

when the field produced by the current carrying moving coil(F_r) tries to come in line with the field produced by the current carrying fixed coil (F_m), a deflecting torque is exerted on the moving  system.

Principle:

Construction:

Working Principle: In case of AC system