The power in a circuit at any instant can be obtained by the product of the current in the circuit and the voltage across its terminals at that instant. If the resistance is known then P = I2R = V2/R
A Wattmeter is a combination of an ammeter and a voltmeter. It consists of two coils known as current coil and potential coil. To measure the current, the current coil is placed in series with a high non-conductive resistance ‘R’ as shown in figure
The reading of a Wattmeter is proportional to
- Current flowing through current coil
- Potential difference across potential coil
- Cosine of the phase angle between voltage and current