ELECTROMAGNETS. MAGNETISM OF IRON. 95 



Now the magnetomotive force & along the rod is equal to 3il 

 where / is the length of the rod. Substituting, therefore, y 

 for df in equation (2) we have : 



This equation may be rewritten thus: 



in which <$ is written for - - . That is, 



The quantity $ is called the reluctance of the magnetic circuit, 

 and the reciprocal of the permeability of the iron, i/n, is called 

 its specific reluctance or reluctivity. 



A portion of a magnetic circuit one centimeter in length 

 (/ = i), and one square centimeter in sectional area (s = i), 

 made of a material having a permeability of unity (/* = i, which 

 is the value of AI for air), has unit reluctance. The name 

 oersted has been adopted for this unit of reluctance by the 

 American Institute of Electrical Engineers. 



Equation (4) is exactly similar in form to the equation express- 

 ing Ohm's law, namely, I = EjR\ and equation (5) is similar in 

 form to the equation for calculating the resistance of a wire, 

 having given its length and section and the specific resistance or 

 resistivity of its material. This analogy between the magnetic 

 circuit and the electric circuit is, however, physically incomplete. 



To find the magnetomotive force required to produce a specified 

 magnetic flux, using equations (4) and (5), proceed as follows: 

 Divide the total flux by the sectional area of each portion of the 

 magnetic circuit thus finding the flux density for each portion. 

 Knowing the flux density <$ for each portion of the circuit, 



