THE MAGNETIC CIRCUIT ELECTROMAGNETS 13 



Magnetic Flux. The unit of magnetic flux is the maxwell; 

 it should be considered as a tube of induction having an ap- 

 preciable cross-section which may vary from point to point. The 

 expression "magnetic lines," which is customary and convenient, 

 should suggest the center lines of these small unit tubes of in- 

 duction. The total number of unit magnetic lines through a 

 given cross-section will be denoted by the symbol $. 



Flux Density. The unit of flux density is the gauss; it is a 

 density of 1 maxwell per centimeter of cross-section. Thus, if 

 A is the cross-section, in square centimeters, of a magnetic 

 circuit carrying a total flux of $ maxwells uniformly distributed 

 over the section, the flux density is B = 3?/A gausses. The 

 symbol B will be used throughout to denote gausses. 



Permeability. What may be thought of as the magnetic 

 conductivity of a substance is known as permeability and rep- 

 resented by the symbol /u. Unlike electrical conductivity, it 

 is not merely a physical property of the substance, because 

 in the case of iron, nickel, and cobalt it depends also upon the 

 flux density. For practical purposes, the permeability of all 

 substances, excepting only iron, nickel, and cobalt, is taken as 

 unity. Permeability can, therefore, be defined as the ratio of 

 the magnetic conductivity of a substance to the magnetic con- 

 ductivity of air. 



Reluctance and Permeance. Magnetic permeanace is the re- 

 ciprocal of reluctance; a knowledge of the permeance of the 

 various paths is useful when considering magnetic circuits in 

 parallel, while reluctance is more convenient to use when making 

 calculations on magnetic paths in series. The reluctance of a 

 path of unit permeability is directly proportional to its length 

 and inversely proportional to its cross-section. Thus, 



Reluctance of magnetic path in air = j 

 Reluctance of magnetic path in iron = j 



If the dimensions are in centimeters, the reluctance will be 

 expressed in oersteds. 



\ MA 



I 



Permeance 



reluctance 



When calculating reluctance or permeance for use in the fun- 

 damental formulas (1) or (2) it is important to express all 



