ALTERNATING CURRENT MEASUREMENTS. 47 



current generator cannot be determined by a voltmeter and an 

 ammeter. 



The wattmeter is almost universally employed for measuring 

 power in alternating-current circuits. 



Power factor. Let the effective value of the electromotive 

 force of an alternator as measured by a voltmeter be E, let the 

 effective value of the current delivered by the alternator as 

 measured by an ammeter be /, and let the actual power deliv- 

 ered by the alternator as measured by a wattmeter be P. The 

 product El is called the apparent power, and it is customary 

 to specify the value of El in volt-amperes in order to avoid 

 the suggestion of actual power which would be conveyed by 

 specifying the value of El in watts. The ratio of the actual 

 power P to the apparent power El is called the power factor 

 of the receiving circuit ; representing the power factor of the 

 receiving circuit by /, we have 



P-Elp (2) 



The power factor of a receiving circuit depends not only upon 

 the nature of the receiving circuit, but also upon the frequency 

 and wave shape of the electromotive force of the alternator. 

 Thus, the power factor of an electric arc between carbon rods is 

 usually about 0.85, that is, the actual power delivered to the arc 

 is only about 85 per cent, of the apparent power EL 



19. The wattmeter. The standard method for measuring the 

 power delivered by an alternator is by means of the wattmeter. 

 The wattmeter is a special form of electrodynamometer, the con- 

 nections of which are shown in Fig. 38. A fixed coil of coarse 

 wire B is connected in series with the receiving circuit to which 

 the power to be measured is delivered, and a suspended or piv- 

 oted coil A of fine wire is connected across the supply mains in 

 series with a non-inductive resistance R. The total current 

 which is delivered to the receiving circuit flows through the fixed 

 coil B, a current proportional to the supply voltage flows 

 through the pivoted coil A, and the force action between the 



