CHAP. 1] FLUX AND MAGNETOMOTIVE FORCE 11 



turns. If the diameter of the ring is kept the same but the cross- 

 section of the path is increased twice, the flux is doubled with the 

 same magnetomotive force. These and similar experiments show 

 that the reluctance and the permeance of a uniform magnetic path 

 obey the same law as the resistance and the conductance of a con- 

 el iu -tor, or the elastance and the permittance of a prismatic slab 

 of a dielectric. 



We can therefore put 



(4) 



where I is the mean length of the path, A is its cross-section, and 

 v is a physical constant. By analogy with resistivity and elastiv- 

 ity, v is called the reluctivity of a magnetic medium. If (R is in rels, 

 and the dimensions of the circuit are in centimeters, v is in rels per 

 centimeter cube. In other words, the reluctivity of a magnetic 

 medium is the reluctance of a unit cube of this medium when the 

 lines of force are parallel to one of the edges. For air and all other 

 non-magnetic substances the experimental value of v is 0.8 rel per 

 centimeter cube, 1 or 0.3 13 rel per inch cube. 



The expression for permeance corresponding to eq. (4) is 



........ (5) 



where the coefficient // is called the permeability of the magnetic 

 medium. It corresponds to the electric conductivity 7- and the 

 dielectric permittivity *. Since the permeance of a path is the 

 reciprocal of its reluctance, the permeability of a medium is the 

 reciprocal of its reluctivity, or 



/<=1A ......... (6) 



\\lirn the perm and the centimeter are used for the units of per- 

 meance and length, permeability is expressed in perms per < 

 meter cube. For all non-magnetic materials /i=1.2f> perms per 

 centimeter cube (more accurate \\ 'it h t lie henry and the 



centimeter as units /i- 1.257 X10~ 8 henri. timeter cube. 



In the English system /< = 3.19 perms per inch cube for non- 



1 More accurately 0.796 rel per centimeter cube. As a rule, magnetic 

 calculations are much less accurate than electrical calculations, because there 

 is no "magnetic insulator" known, so that there is always some magnetic 

 leakage present, which is dillirult to take into consideration. For this reason 

 the value 0.8 is sufficiently accurate for most practical purposes. 



