CHAPTER VIII 

 THE TRANSFORMER 



68. Principles Involved. A transformer is a piece of 

 stationary apparatus by means of which a-c. power may 

 be changed from one voltage to another. It consists essen- 

 tially of a closed magnetic circuit (made of laminated iron 

 and called the core) on which are placed two coils of insu- 

 lated wire, the two coils being entirely separate and insu- 

 lated from one another. (This later qualification will be 

 modified when discussing a special type called the auto- 

 transformer.') The two coils generally have a widely dif- 

 ferent number of turns; a certain transformer, for instance, 

 has 93 turns in one coil and 1800 in the other. 



Exciting Current. One of the coils (called the primary) 

 is connected to an a-c. line of suitable voltage and the 

 resulting current which flows (the other coil being open 

 circuited) serves to magnetize the iron core; it is called the 

 exciting current. The magnetic flux through the core will 

 evidently be an alternating one, being in one direction 

 with positive current and in the reverse direction when the 

 current has reversed; hence any turn of wire surrounding 

 the core will have induced in it an e.m.f. because of this 

 varying flux. 



Induced E.M.F. in the Secondary Coil. The second coil 

 (the one to which power is not supplied, called the secondary) 

 is also wound on the core and therefore will have an e.m.f. 

 induced it. Lamps, motors, etc., may be connected to 

 this secondary coil and will be supplied with power just 

 as well as though they were connected to the line which 



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