Wattmeter:

It is an instrument that measures DC power or true AC power. The wattmeter uses fixed   coils   to   denote   current, although the movable coil denotes voltage that was display in the figure.  Coils L_I1 and L_I2 are the fixed coils in series along with one another and serve as an ammeter.  The two I terminals are linked in series with the load. A movable coil Lv, and its multiplier resistor R_s, are used as a voltmeter, along with the V terminals connected in parallel with the load. A meter deflection is proportional to the V_I that is power.

Figure: Wattmeter Schematic

Wattmeters are rated in terms of their maximum voltage, current, and power.  All of these ratings must be observed to avoid damage to the meter.

Given Equation (14-15) is the mathematical representation of calculating power within a DC circuit.

P = VI or P = I²R                                                                            (14-15)

Given Equation (14-16) is the mathematical representation for calculating power in an AC circuit.

P = V_RmsI_Rmscosθ       or P = I²R                                                        (14-16)