368 ELEMENTS OF ELECTRICAL ENGINEERING. 



is $, so that the flux density in the air gap is to be multiplied 

 by the length in centimeters across the gap to give the magneto- 

 motive force required for the gap. 



(b) To find the flux produced by a specified magnetomotive 

 force. In the case of a rod of uniform size and quality this 

 problem (b) is simply the reverse of problem (a). When, how- 

 ever, the parts of a magnetic circuit are of different sizes or of 

 different materials, the flux produced by a specified magnetomo- 

 tive force is best determined as follows : Calculate, as explained 

 under (a), the magnetomotive force required to produce a series 

 of arbitrarily assumed values of flux. Arrange these results in 

 tabular form, plot them, and trom this plot find the flux corre- 

 sponding to the prescribed magnetomotive force. 



15. Analogy between the magnetic circuit and the electric circuit. 

 Definition of magnetic reluctance. The statements given in Art. 

 1 4 (a) and (b) are complete statements of the fundamental princi- 

 ples and methods of calculation of the magnetic circuit. A slight 

 modification of the fundamental methods outlined in Art. 14 (a) 

 and (b) may be used. This modified method is based upon an 

 analogy between the magnetic circuit and the electric circuit. 

 It, however, contains no physical or mathematical principles in 

 addition to those involved in the fundamental method outlined 

 in Art. 14 (a) and (b), and its only advantage is that the funda- 

 mental equation (9) is rearranged so as to correspond exactly 

 in form to the equation for Ohm's law. 



The fundamental equation 



=fjiM (i) 



when applied to an iron rod which forms a magnetic circuit, may 

 be transformed as follows : The magnetic flux 4> through the rod 

 is equal to eBs, whence equation (i) may be written : 



Now the magnetomotive force & along the rod is equal to &CI 

 where / is the length of the rod. Substituting therefore &\l for 

 &C in equation () we have : 



