Correct option is D
The wattmeter is an instrument for measuring the electric active power (or the average of the rate of flow of electrical energy) in watts of any given circuit.
A wattmeter consists of a current coil and a potential coil.
The wattmeter is a combination of an ammeter and a voltmeter.
It measures both voltage and current and determines the resultant power.
Electric power is measured using an instrument called a wattmeter.
The wattmeter comprises two coils-one for current and the other for voltage.
Types of Wattmeters:
o Electrodynamometer wattmeter - for both DC and AC power measurement
o Induction wattmeter - for AC power measurement only.
The electrodynamometer wattmeter works on the dynamometer principle i.e. a mechanical force acts between two current carrying conductors or coils.
o The fixed coil is called current coil while the moving coil is called potential coil.
The induction type wattmeter can be used to measure AC power only.
o The working of induction type wattmeter is based on the principle of electromagnetic induction.
