The instrument most commonly used for power measurement is the dynamometer.
It is built by (1) two fixed coils, connected in series and positioned coaxially with space between them, and (2) a moving coil, placed between the fixed coils and equipped with a pointer (Figure 3.2(a)).
The torque produced in the dynamometer is proportional to the product of the current flowing into the fixed coils times that in the moving coil.
The fixed coils, generally referred to as current coils, carry the load current while the moving coil, generally referred to as voltage coil, carries a current that is proportional, via the multiplier resistor R , to the voltage across the load resistor R .
As a consequence, the deflection of the moving coil is proportional to the power dissipated into the load.