THE MAGNETIC CIRCUIT ELECTROMAGNETS 11 



circuit, will be most useful to the designer of electrical machinery. 

 The magnetic flux is thought of as consisting of a large number of 

 tubes of induction, each tube being closed upon itself and linked 

 with the electric circuit to which the magnetic condition is 

 due. The distribution of the magnetic field will depend upon the 

 shape of the exciting coils, and upon the quality, shape, and 

 position, of the iron in the magnetic circuit. The amount of 

 the magnetic flux in a given magnetic circuit will depend upon 

 the m.m.f. (magnetomotive force) and therefore on the current 

 and number of turns of wire in the exciting coils. 



OHM'S law for the electric circuit can be put in the two forma- 



te) 



Current = 



^ 



resistance 



, or I = 



~ 

 R 



(b) Current = e.m.f. X conductance, or / = E X ( ) 



Similarly, in the magnetic circuit: 



magnetomotive force 



- 



magnetic reluctance 



(a) Magnetic flux of induction = 



,_5-t. a, 



(6) Magnetic flux = magnetomotive force X permeance 

 or $ = m.m.f. X P (2) 



In this analogy, $ is the total flux of induction, usually ex- 

 pressed in C.G.S. lines, or maxwells; m.m.f. is the force tending 

 to produce the magnetic condition expressed in ampere-iitms 

 (the engineer's unit) or in gilberts the C.G.S. unit; and reluctance 

 is the magnetic equivalent of resistance in the electric circuit. 

 It is necessary to bear in mind that although these are funda- 

 mental formulas of the greatest value in the calculation of mag- 

 netic circuits, yet they are based on an analogy which, with 

 all its advantages, has its limitations. The chief difference 

 between OHM'S law of the electric circuit and the analogous 

 expression as applied to the magnetic circuit lies in the fact that 

 the magnetic reluctance does not depend merely upon the 

 material, length, and cross-section, of the various parts of the 

 magnetic circuit, but also when iron is present on the amount 

 of the flux, or, more properly, on the flux density, which is an 

 important factor in the determination of the permeability GO- 



