THE TRANSFORMER. 



195 



netic circuit is called the transformer core, although it may have 

 a shape more suggestive of a shell than of a core. 



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

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

 the coil of few turns is the pri- 

 mary coil, the transformer takes 

 large current at low electromo- 

 tive force and delivers small 

 current at high electromotive 

 force. This is called step-up 

 transformation. 



When the coil of many turns 

 is the primary, the transformer 

 takes small current at high elec- 

 tromotive force and delivers 

 large current at low electromo- 

 tive force. This is called step- 

 down transformation. The ob- 

 ject of step-up and step-down 

 transformation is explained in Fig> 162 . 



Article 3. 



Mounting of transformers. Transformers are usually enclosed 



Fig. 163. 



in metal cases, and the case is usually filled with oil ; the pur- 

 pose of the oil being partly to improve the insulation of the coils 



