352 ELEMENTS OF ELECTRICAL ENGINEERING. 



(/;) Bunched ivinding. The winding of wire may be bunched 

 at one or more places along the iron core. In this case the mag- 

 netic field which the winding tends to produce in the region occu- 

 pied by the iron core varies greatly in intensity from point to point 

 in the core, and the direction of this field is not parallel to the rod 

 at each point. The magnetizing action of such a field upon an 

 iron rod is discussed in the next article. 



Electromagnets usually have bunched windings. 



2. The magnetizing action of a bunched winding. Definition 

 of magnetomotive force. The magnetizing action upon an iron 

 rod of a non-uniform magnetic field, such as the magnetic field 

 produced by a bunched winding, depends upon the average value, 

 along the rod, of the component of the magnetizing field parallel 

 to the rod. The product of this average value, into the length of 

 the rod is called the magnetomotive force along the rod. That is : 



&=IA (i) 



in which & is the magnetomotive force along a rod in c. g. s. 

 units, / is the length of the rod in centimeters, and A is the aver- 

 age value along the rod, of the component of the magnetizing 

 field parallel to the rod. The rod may be straight or curved. 



A rod passing through a magnetic field determines a certain 

 line or path through the field, and we speak of the magnetomotive 

 force along this path, whether the rod is there or not. 



3. Proposition. The magnetomotive force along a path in a 

 magnetic field is equal to the work done by the magnetic field upon 

 a unit magnet pole while the pole is made to travel along the path, 

 or ; the magnetomotive force along the path is equal to W/m 

 where W is the work done by the field upon a pole of strength 

 m, while the pole is made to travel along the path. That is : 



*-- w 



m v } 



in which eMs the magnetomotive force along a path in a magnetic 



