354 



ELEMENTS OF ELECTRICAL ENGINEERING. 



4. Magnetomotive force of a coil. The most important case of 

 magnetization of an iron rod or core by a bunched winding of 

 wire is the case in which the core is in the form of an endless rod 

 which passes through, or links with, the coil as shown in Fig. 2. 

 The magnetomotive force along the rod in this case is entirely 



con. 



Fig. 2. 



Fig. 3. 



independent of the shape and length of the rod and of the 

 shape and size of the coil. It depends only upon the number of 

 turns of wire in the coil and upon the strength of the current in the 

 wire. 



This magnetomotive force along an endless rod which links 

 with a coil of wire in which an electric current is flowing, is called 

 the magnetomotive force of the coil. 



Examples. Fig. 3 shows a two-pole dynamo with two field 

 coils ZZ. The field magnet iron of this dynamo together 

 with the iron of the armature constitute a nearly endless rod or 

 core of iron which links with both field coils, and which consti- 

 tutes what is called the magnetic circuit of the dynamo. 



Fig. 4 is a sketch of a four-pole dynamo with four field coils 

 ZZZZ. The iron of the field magnet and armature presents 

 in this case four distinct magnetic circuits, as indicated by the 

 dotted lines. Through each of these magnetic circuits flux is 

 forced by the combined magnetomotive force of the two field 

 coils through which the circuit passes, that is, with which it 

 links. 



