ADVANCED ELECTRICITY AND MAGNETISM. 



Iron rod 



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

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

 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. 55. 

 The magnetomotive force along the rod in this 

 case is entirely independent of the shape and 

 length of the rod and of the shape and size of 

 the coil. It depends only upon the num- 

 ber of turns of wire in the coil and upon 

 the strength of the current in the wire. 

 This definite magnetomotive force along 

 an endless rod which links with a coil of 



Fig. 55. 



wire in which an electric current is flowing, is called the mag- 

 netomotive force of the coil. 



Examples. Fig. 56 shows a two-pole dynamo with two field 

 coils ZZ. The field-magnet iron of this dynamo and the iron 

 of the armature together constitute a 

 nearly endless rod or core of iron which 

 links with both field coils, and which 

 constitutes what is called the magnetic 

 circuit of the dynamo. 



Figure 57 is a sketch of a four-pole 

 dynamo with four field coils ZZZZ. 

 The iron of the field magnet and arma- 

 ture 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 forces of the two field coils through which the 

 magnetic circuit passes, that is, with which it links. 



The total magnetomotive force acting on any magnetic circuit 

 is equal to the sum of the magnetomotive forces of the coils 

 with which the circuit links. 



49. Proposition. The magnetomotive force of a coil is given 

 by the equation : 



Fig. 56. 



