184 THE TRANSFOEMER. , 



lags 90 behind the voltage, and partially energy current 

 necessary to overcome the iron losses and the small copper 

 losses due to the no-load current. 



In the case represented in Fig. 85 the magnetising current 

 is -8 amperes, and is represented by the line C m drawn to a 

 scale of amperes. 



The current C m is the portion of the current which 

 alone acts as magnetising current and the magnetic flux 

 produced in the transformer core is given by the formula on 



Z> 7 



page 140, viz., ct = -- x 1-435. 

 f* 



Consequently, the number of linss actually formed in 

 the core at any instant is proportional to the instantaneous 

 value of the magnetising current at the same instant. Thus 

 we may represent the field strength by a vector line drawn 

 parallel to the magnetising current, to a scale of " number of 

 lines." This is shown as the line F. 



In the secondary circuit there will be induced a voltage 

 equal to the rate of change of lines x number of secondary 

 turns -|- 10 8 . 



Since the voltage depends upon the rate of change, and not 

 on the number of lines, it will lag in phase J- period 

 behind -the line showing " number of lines." It must be 

 remembered that the primary induced back voltage and the 

 secondary induced voltage are in phase, being due to the same 

 cause. Thus the secondary voltage will be represented by 

 a line exactly opposite in direction to the line of primary 

 applied volts overcoming the back electromotive force. The 

 secondary volts would be represented by E., one-tenth 

 of the length of E\ if the ratio of transformation is 10 to 1. 



For convenience in drawing, it is the usual plan to 

 represent the primary and secondary voltages not of the 

 actual relative magnitude, but to show the low-tension 

 voltage as multiplied by the ratio of transformation, thus 

 bringing it to the same length as the primary voltage on the 

 diagram. In reading the results of a diagram, all lines 

 representing voltages in the secondary circuit must be 

 divided by the ratio of transformation, in order to give the 

 actual number of volts. Adopting this device the line E 2 

 is shown in Fig. 85 as being eqijal in length to E^ 



The secondary voltage is seen from the figure to be always 

 exactly opposite in phase to the applied voltage overcoming the 



