ELECTROMAGNETS. MAGNETISM OF IRON. 369 



This equation may be rewritten thus : 



*-J do) 



in which eft is written for I fp x ljs. That is, 



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

 and the reciprocal of the permeability of the iron, I //*, 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), and made of 

 a material having a permeability of unity (p = I , which is the 

 value of yu, for air), has unit reluctance. The name oersted has 

 been adopted for this unit of reluctance by the American Institute 

 of Electrical Engineers. 



Equation (10) is exactly similar in form to the equation ex- 

 pressing Ohm's law, namely, /= EjR ; and equation (i i) is simi- 

 lar 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, 

 for the magnetic reluctance of an iron circuit and the reluctivity 

 of the iron both increase as the flux increases, whereas the resis- 

 tance of an electric circuit does not vary with the current, unless 

 changes of temperature occur. 



To find the magnetomotive force required to produce a speci- 

 fied magnetic flux, using equations (10) and (n), 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 take the corresponding values of /-c from the table in 

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