66 



POWER AND POWER-FACTOR. 



The construction adopted is based on the fact that the 

 angle contained by a serai-circle is always a right angle. 

 The useful and idle voltage vectors are always mutually 

 perpendicular, and have as a resultant the voltage 

 represented by the diameter of the semicircle. 



Choking Coil. A resistance having a high self-induction 

 may have a high apparent resistance, although its true, or 

 ohmic, resistance is small. It may consequently be used 

 in series with an arc lamp in order to reduce the current 

 to the value required, and answers then the purpose of 

 the "ballast" resistance which is necessary to make the 

 lamp burn steadily. 



Since the voltage absorbed by the self-induction does 

 not represent loss of power (i.e., is not energy voltage), 

 a coil possessing low ohmic resistance and high self- 

 induction may be used more economically than an ordinary 

 resistance in many cases to reduce the current in an 

 alternating circuit. 



The following experiment illustrates the use of a 

 choking coil and also the saving which may be effected by it. 



EXPERIMENT IX. DETERMINATION OF POWER LOST 

 IN A CHOKING COIL AND POWER SAVED BY ITS USE. 



DIAGRAM OF CONNECTIONS. 



FIG. 30. 



I M 2 Source of alternating current. 



X = Arc. 



r = Choking coil 



F! = Voltmeter reading voltage of circuit. 

 V 2 = Voltmeter reading voltage of arc. 



A = Ammeter reading current in circuit. 



W = Wattmeter reading power of circuit. 



S = Switch. 



