94 ADVANCED ELECTRICITY AND MAGNETISM. 



In this kind of a calculation it is to be remembered that flux 

 density in air is c^, so that the flux density in the air gap is to 

 be multiplied by the length in centimeters across the gap to 

 give the magnetomotive 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 magneto- 

 motive force is best determined as follows: Calculate, as ex- 

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

 a series of arbitrarily assumed values of flux. Arrange these 

 results in tabular form, plot them as a curve, and from this plot 

 find the flux corresponding to the prescribed magnetomotive 

 force. 



59. Analogy between the magnetic circuit and the electric 

 circuit. Definition of magnetic reluctance. The statements 

 given in Art. 58 (a) and (b) are complete statements of the funda- 

 mental principles and methods of calculation of the magnetic 

 circuit. A slight modification of the fundamental methods out- 

 lined in Art. 58 is, however, extensively used. This modified 

 method is based upon an analogy between the magnetic circuit 

 and the electric circuit, but it contains no physical or mathe- 

 matical principles in addition to those involved in the funda- 

 mental method outlined in Art. 58, and its only advantage is 

 that the fundamental equation is rearranged so as to correspond 

 exactly in form to the familiar equation for Ohm's law. 



The fundamental equation 



# = M (i) 



as applied to an iron rod which forms a magnetic circuit may be 

 transformed as follows: The magnetic flux $ through the rod 

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



(2) 



