76 ADVANCED ELECTRICITY AND MAGNETISM. 



(b) Bunched winding. 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 produces in the region occupied 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. 



46. The magnetizing action of a bunched winding. Defini- 

 tion 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 and the 

 length of the rod is called the magnetomotive force along the rod. 



That is: ff=lA (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 

 average 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. 



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

 magnetic field is equal to the work per unit pole done by the magnetic 

 field upon a magnet pole while the pole is made to travel along the 

 path', that is, the magnetomotive force along a 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: 



in which & is the magnetomotive force along the path. 



* At each point. 



