2 ELEMENTS OF ELECTRICAL ENGINEERING. 



ing across a north pole an electromotive force is induced in the 

 wire in one direction, and while the wire is sweeping across a 

 south pole an electromotive force is induced in the wire in the 

 opposite direction. This repeatedly reversed electromotive force 

 is called an alternating electromotive force and it produces an 

 alternating current in the wire and in an outside circuit to which 

 the ends of the wire are connected. 



In commercial alternators a large number of wires is used 

 instead of the single wire W shown in Fig. I, and these wires 



JL 



sft O 



__1 J 



To receiving circuit 



Fig. la. 



Fig. 2*. 



are placed in slots in the periphery of a rotating mass of lamin- 

 ated iron. Thus Fig. 2 shows four wires placed in four slots, and 

 Fig. 3 shows sixteen wires placed in sixteen slots. 



Figures 2b and 3^ are what are called developed diagrams 

 which show how the wires are connected together and how they 

 are connected to two insulated metal rings r and r' upon 

 which two brushes a and b rub and keep the moving wires 

 connected to an outside receiving circuit. 



The laminated structure A A with its winding of wire is called 

 the armature of the alternator, and the metal rings r and r' are 

 called collecting rings. 



The field magnet of an alternator is generally an electromagnet 

 which is excited by direct current from a small auxiliary direct- 



