1. Wattmeter

2. A wattmeter is an electro dynamo – meter instrument used to measure the power in a
given circuit .
this type of meter has two coils,one is fixed and the other is free to move .

3. Here, a pointer is attached to the movable coil and the deflection is propotional to the repulsive force generated from the currents moving through the two coils .

4. Here , the movement is similar to the darsonval movement .
When the line current flows through the current coil , a field is set around the coil . the strength of this field is proportial to the current and in phase with it .

5. Note:
the high value of the resistance connected in series with the potential coil makes that potential circuit as purely as possible .

6. Hence :
the part in the potential circuit practically in phase with the line voltage .
therefore , when the voltage is applied to the potential circuit . the circuit is propotional to and in phase with the line voltage .

7. Note:
The mechanical movement causes a deflection that is propotional to the voltage value of the power .
Notice that one end of each coil is usually marked with a ( + ) sign while the other is ( - ) . here an upscale reading is obtained if a ( + ) current is flowing in to the ( + ) end of the current coil while the ( + ) terminal of the potential coil is ( + ) WRT to the end .

8. Example:

9. Solution: Using the mesh analysis analysis: I = ( 11.18 ) / 153.4 A
P = (100)*(11.18)*cos(O-1534)
= -1000W
Using the down scale deflection for a ( + ) reading.