CH. XIII] THE ELECTRIC ARC 535 



of this number, or 15 amperes from the line. The watts in the two 

 cases are theoretically exactly the same. 



The watts for the primary are 220x15 = 33 - 



The watts for the secondary are 55 x 60 = 3300. 



(1) Volts secondary _ Turns secondary 



Volts primary Turns primary 



(2) Amperes primary Turns secondary 

 Amperes secondary Turns primary 



FIG. 291. DIAGRAM OF A TRANSFORMER. 



Two coils of a wire, Primary and Secondary, are wound on an iron ring. An 

 alternating current in the primary sets up an alternating magnetic flux in the 

 iron ring, which in turn sets up an alternating electric potential in the secondary 

 coil. 



THE ELECTRIC ARC 



740. The construction of an electric arc is very simple. Two 

 electrodes are taken which may be made of any conducting material. 

 One electrode is connected directly to one of the wires of a direct 

 current supply of over 40 volts, the other electrode is connected 

 through a rheostat to the other wire (fig. 280). When the two 

 electrodes are brought in contact an electric current will flow 

 between them. If now, the electrodes are slightly separated, the 

 current will not be immediately interrupted, but will flow through 

 the air gap between the electrodes. 



