THE TRANSFORMER. 203 



force of the alternator which is forcing current through the primary 

 coil. Resistance of primary coil neglected. 



When the secondary coil is on open circuit, just enough cur- 

 rent flows through the primary coil to produce the degree of 

 magnetization above specified. Let this value of the primary 

 current, which is called the magnetizing current, be represented 

 by M. When current /" is taken from the secondary coil, 

 additional current /', called the load current, flows through 

 the primary coil. The current M still suffices to magnetize the 

 core, and the magnetizing action of I" is exactly neutralized by 

 the equal and opposite magnetizing action of /' '. The magnetizing 

 action of I" is measured by the product N"I" y and the mag- 

 netizing action of I' is measured by the product N' I' , so that, 

 ignoring algebraic signs, we have 



NT = N"I" 

 or /' N" 



7 Tr = W r 



Ratio of primary electromotive force to secondary electromotive 

 force. The rapid reversals of magnetization of the iron core in- 

 duce a certain electromotive force a in each turn of wire sur- 

 rounding the core. Therefore the total electromotive force 

 induced in the primary coil is N 1 a. This is the reacting electro- 

 motive force in the primary coil and it is equal and opposite, as 

 pointed out above, to the electromotive force R f which is push- 

 ing current through the primary coil ; so that, ignoring signs, we 

 have 



E = N'a 



Similarly, the total electromotive force, E f , induced in the 

 secondary coil is 



E' = N"a 

 Therefore 



E ' N ' ^ 



W' = W' 



*The above discussion should in strictness refer, primarily, to instantaneous values 

 of /' and 7 X/ and to instantaneous values of E / and E ff . Thus i f and i f ' 



