CHAPTER VI 

 THE MEASUREMENT OF POWER 



In direct-current circuits the measurement of power, or rate 

 of expenditure of electrical energy, is best accomplished by the 

 use of calibrated ammeters and voltmeters. In alternating- 

 current circuits where both the electromotive force (or potential 

 difference) and the current are varying from instant to instant, 



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FIG. 175. Illustrating instantaneous and average power. 



passing through a definite cycle of values, the " instantaneous 

 power," or the power being given to the circuit at a particular 

 instant is vi. The power to be measured is the average value 

 of vi during the cycle, 



1 C T 

 P = vidt-. 



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