INDUCED ELECTROMOTIVE FORCE. 125 



delivers what is called an alternating current, that is, a current 

 which is subject to rapid periodic reversals of direction. The 

 direct-current generator, on the other hand, delivers a current 

 which is not reversed in direction and which is usually quite 

 steady in value. 



68. The alternating-current dynamo. The simplest form of 

 the alternating-current dynamo is shown in Fig. 83. A wire W, 



Fig. 83. 



perpendicular to the plane of the paper, is moved sidewise along 

 the dotted line so as to cut the magnetic lines of force which 

 emanate from the inwardly projecting poles NSNS of a large 

 electromagnet which is called the field magnet of the alternator. 

 While the wire is sweeping across a north pole an electromotive 

 force is induced in it in one direction, and while the wire is sweep- 

 'ing across a south pole an electromotive force is induced in it 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 a wire may be connected. 



In commercial alternators large numbers of wires are used in- 

 stead of the single wire W shown in Fig. 83, and these wires 

 are placed in slots in the periphery of a rotating cylindrical mass 

 of laminated iron. Thus, Fig. 84 shows 4 wires in 4 slots and 



