46 ELECTRICAL ENGINEERING 



Thus the difference of magnetic potential between two points is 

 the line integral of the magnetic force between them. 



35. Magnetomotive Force. The difference of magnetic 

 potential or the line integral of the magnetic force between two 

 points is called the magnetomotive force (m.m.f.) between the 

 points. It causes magnetic flux to pass from one to the other. 



Unit m.m.f. will produce one line of magnetic flux through a 

 (cm.) 3 of air and /* lines through a (cm.) 3 of a magnetic material 

 of permeability /*. It is called the gilbert. 



Since magnetomotive force is the line integral of the magnetic 

 force or 



M 



= facdr 



therefore the magnetic force is the space rate of change of m.m.f. 

 or 



. 



(72) 



and thus magnetic force is the m.m.f. per centimeter. 



36. Permeability. Permeability is the ratio of the magnetic 

 conductivity of a substance to the magnetic conductivity of air 

 and is represented by /*. 



Lines of magnetic flux pass through air or any other substance 

 except iron, nickel or cobalt, as easily as they do through a 

 vacuum. The permeability of such substances is for all practical 

 purposes the same and is taken as unity. Iron and its compounds 

 and to a lesser degree nickel and cobalt are found to allow mag- 

 netic lines to pass through them much more easily than empty 

 space; that is, a given m.m.f. will produce a much larger flux 

 through a volume of iron than it will through an equal volume of 

 air. The permeability of the iron is therefore greater than that 

 of the air and is expressed by some number greater than unity. 



The permeability of magnetic materials is not constant but 

 varies with the induction density as shown in Art. 49. 



37. Magnetic Reluctance. The reluctance of a magnetic 

 circuit may be defined as the resistance offered by the circuit to 

 the passage of magnetic flux through it and is represented by 91. 



When unit m.m.f. is applied across a (cm.) 3 of air one line of 

 magnetic flux is produced crossing from one face to the other, 

 or one line of magnetic induction per square centimeter is pro- 

 duced through a length "of one centimeter. The reluctance of a 



