THE TRANSFORMER 



285 



leg, on which the coils are placed. This type uses con- 

 centric, cylindrical coils (which are the easiest to manu- 

 facture), is compact in its arrangement, and is more easily 

 cooled than either of the other types. A sketch of the trans- 

 former is given in Fig. 179; it shows how freely the cooling 

 oil may circulate between the coils and around all parts. 



Size of Wire in Transformer Coils. The size of wire 

 used on the two coils is not the same; it is much smaller 

 for the high-voltage coil than for the low-voltage coil. The 



Primary - 

 Coils r 



Core plate 



Core plate 



1 Secondary 

 Coils 



FIG. 177. The Laminations, in the Form of Hollow Rectangles, are 

 Split so That They May be Put around the Sides of the Coils. 



most efficient design is one in which the I 2 R in the primary 

 coil is equal to that in the secondary. We have shown in 

 equation (72) that l2/Ii=Ei/E2, or, in other words, the 

 currents in the two coils are in the inverse ratio of their 

 voltages. Let us put 



Then 



= a, the ratio of the transformer. 

 = al\ and if we are to have 

 = Ii 2 Ri, then evidently 



(74) 



If for example a transformer has a ratio of 20:1, the high- 

 voltage coil should have 400 ( = 20 2 ) times as much resist- 



