134 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



that the iron core of the induction coil is not a complete magnetic 

 circuit, but has magnet poles at its ends. The effect of these 



""IS 



~ iron' 



Fig. 93. 



Fig. 94. 



magnet poles is to facilitate the quick demagnetization of the core 

 when the primary circuit of the induction coil is broken. 



The action of the transformer. Alternating current is supplied 

 to either coil of the transformer. This alternating current pro- 

 duces rapid reversals of magnetization of the iron core. These 

 magnetic reversals induce an alternating electromotive force in 

 the other coil which delivers alternating current to any circuit to 

 which it may be connected. The coil of a transformer which re- 

 ceives alternating current is called the primary coil, and the coil 

 which delivers alternating current is called the secondary coil. 



Step-up and step-down transformation. Usually, one coil of a 

 transformer has many more turns of wire than the other. The 

 coil of many turns may act as the primary coil, taking a small 

 current at high electromotive force from an alternator ; and in 

 this case the coil of few turns will be the secondary coil, and it 

 will deliver a large current at low electromotive force to a receiv- 

 ing circuit. This action is called step-down transformation. 



The coil of few turns on the other hand may act as the primary 

 coil, taking a large current at low electromotive force from an 

 alternator; and in this case the coil of many turns will be the 

 secondary coil and it will deliver a small current at high voltage 

 to a receiving circuit. This action is called step-up transformation. 



The object of step-up and step-down transformation may be 

 explained as follows : The transmission of a given amount of 



