206 nature's flMraclee. 



It will be seen from the foregoing that these 

 transformers have to be made with reference 

 to the use the current is to be put to. In gen- 

 eral shape they are alike in appearance, the 

 difference being chiefly in the relation the pri- 

 mary sustains to the secondary coils. Then- 

 is another kind of transformer that is used 

 when it is necessary to have the current always 

 running in the same direction. This trans- 

 former, as heretofore explained, does not 

 change the voltage of the current, but simply 

 transforms what was an alternating into a di- 

 rect current. By alternating current we mean 

 one that is made up of impulses of alternat- 

 ing polarity first a positive and then a nega- 

 tive. The direct current is one whose impulses 

 are all of one polarity. The direct current is 

 required for all purposes where electrolysis 

 (chemical decomposition by electricity, as of 

 silver for silver-plating, etc.) is a part of the 

 process. The alternating current may be used 

 without transformation in all processes where 

 heat is the chief factor. For motive power 

 either current may be used, only the electro- 

 motors have to be constructed with reference 

 to the kind of current that is used. 



The rotary transformer, which may be 

 driven by any power, consists of a wheel car- 

 rying a rotating commutator so arranged with 

 reference to brushes that deliver the current 

 to the commutator and carry it away from 



