58 



POWER AND POWER-FACTOR. 



The deflection of the instrument is produced by the 

 mutual action of these two coils, so that it is proportional 

 to the product (amperes x volts). Since the inertia of the 

 moving parts of the instrument is far too great to allow them 

 to follow the rapid variations occurring at every period, the 

 deflection is steady, and is proportional to the mean value of 

 the power in the circuit. 



Power Factor. It was pointed out that in Fig. 25 the 

 curves of current and voltage are not in phase, since they 

 do not pass through their zero values simultaneously. In 

 consequence of this a portion of the power represented by the 

 product (current x volts) is always negative during each period. 



FIG. 25. CURVES OF .CURRENT, VOLTAGE, AND WATTS. 

 Angle of lag =30*. 



If the same current and voltage existed in a circuit without 

 self-induction there would be no difference in phase between 

 them, and the two curves would always pass through zero 

 at the same time, and at every instant would both be on the 

 same side of the horizontal axis. Hence in this case their 

 product would always be positive, and the average useful 

 power of the circuit would be correspondingly greater than 

 is the case for the condition illustrated in Fig. 25, where the 

 power is partly positive and partly negative. 



