210 ELECTRICAL MACHINERY 



|X 



which says that the power used up in the circuit is equal 

 to the product of the (current) 2 and a certain constant, R, 

 which we call the effective resistance. This equation con- 

 stitutes the definition of the term effective resistance. 

 From the definition it is evident that the effective resistance 

 of any circuit may be obtained by dividing the wattmeter 

 reading of the circuit by the squared value of the ammeter 

 reading. 



Now a wattmeter reads, for any circuit, El cos <!>, and 

 E cos cj>, is evidently the component of the impressed force 

 which is in phase with the current. But if 



Watts = PR = EI cos (j>, 

 it is evident that 



IR=Ecos<b (45) 



which gives us another definition for effective resistance; 

 it is that quantity which multiplied by the current, gives 

 the component of the impressed force used up in phase 

 with the current, or active voltage. 



Effective Resistance Increased by Hysteresis Loss, Radia- 

 tion Loss, etc. Let us suppose a circuit consists of a coil 

 of wire around an iron core and that an alternating current 

 is flowing through the coil; the iron core, being magnetized 

 first in one direction and then in another, will have to take 

 in enough energy to supply the hysteresis and eddy 

 current losses. This energy must be supplied by the 

 electric circuit which is magnetizing the core and if a watt- 

 meter is placed in the circuit it will indicate not only the 

 power used in heating the wire of which the magnetizing 

 coil is made, but also that used in heating the iron core. 



Hence, when the resistance of such a circuit is deter- 

 mined by dividing the wattmeter reading by the (current) 2 

 it will evidently be more than the conductor resistance by 

 an amount depending upon the magnitude of the loss in 



