Checking Wattmeter Power Loss
The stationary (current) coils and the moving (voltage) coil of a wattmeter have resistance, resulting in some circuit power loss by the wattmeter. Unless this power loss is considered, incorrect power readings will result.
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If you wish to find power dissipated in an electrical load, measure any two of the three basic electrical quantities- current, voltage, and resistance. For example, you will recall that power can be calculated by multiplying voltage by current: P = VI. Therefore, if you use a voltmeter to measure the voltage across a load, and a current meter to measure the current flowing through the load, insert these values into the power equation. Similarly, you can measure current through the load and the resistance of the load, and then calculate power with: P = 12 R. Or you can measure the voltage across the load and use the equation: p = y2; R.
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