48 



ELECTRICAL ENGINEERING 



Assuming the flux to be unity in the last equation the reluc- 

 tance of the path may be defined as the m.m.f. required to pro- 

 duce unit flux through it. 



38. Permeance. The permeance of a magnetic path is the 

 reciprocal of its reluctance and is represented by 2; thus 



M 



(77) 



and assuming that the m.m.f. acting is unity, the permeance 

 may be defined as the flux through the path produced by unit 

 m.m.f. 



The permeance of a path of uniform section is 



I 



(78) 



and is directly proportional to the sectional area and to the per- 

 meability, and is inversely proportional to the length of the path. 



39. Electromagnetics. The region surrounding a conductor 

 carrying a current of electricity is a magnetic field, A current 

 of electricity therefore represents a magnetomotive force. If the 

 conductor is isolated from other magnetic forces the lines of 

 force will form circles around it. 



Maxwell's Corkscrew Rule. The direction of the current and 

 that of the resulting magnetic force are related to one another 

 as the forward travel and the twist of an ordinary corkscrew. 

 This rule is illustrated in Fig. 39. 



FIG. 39. Magnetic flux produced by electric current. 



The symbol represents a current flowing down and a 

 current flowing up. 



Faraday discovered that a current is induced in a closed coil 

 of wire when a magnet is brought near it. The same effect is 

 noticeable if a coil of wire carrying current is moved to or from 



