370 ELEMENTS OF ELECTRICAL ENGINEERING. 



Art. 1 2. Knowing the value of /*, the length /, and the sectional 

 area s of each portion of the circuit, calculate the magnetic re- 

 luctance $1 of each portion. Add these separate reluctances 

 together to get the total reluctance of the entire circuit and 

 multiply the total flux <I> by this total reluctance to get the re- 

 quired magnetomotive force cf y according to equation (10). 



16. Two typical examples of magnetic circuit calculations, ig- 

 noring magnetic leakage. 



(a) Calculation of the amount of field excitation required for a 

 lifting magnet, design of magnet and weight to be lifted being 

 given. 



Fig. 12 is a sketch of a lifting magnet of which the dimen- 

 sions and material of each part is given. The dotted line repre- 

 sents the middle line of the magnetic circuit and the length of 

 this line in each part of the circuit may 

 be taken as the mean length / of that part. 

 The flux through the cores CC is so nearly 

 uniform that the sectional area of each core 

 is accurately the sectional area s of the 

 stream of flux in that part of the circuit. 

 The flow of flux through yoke Fand arma- 

 ture A is not uniform, but the actual sec- 

 tional areas of yoke and armature at a and b represent the mean 

 sectional areas of the flux stream in yoke and armature nearly 

 enough for practical purposes. The length of each air gap (par- 

 allel to flux stream of course) is pretty definite, but the flux 

 spreads out greatly where it crosses an air gap and therefore the 

 sectional area of the air gap is indeterminate. When the air gap 

 is short, as in Fig. 12, it is sufficient to take the area of the 

 smaller face (the ends of the cores in Fig. 12) as the sectional 

 area of the air gap. 



Let s be the sectional area of each air gap in Fig. 1 2 and < the 

 flux crossing each gap. Then the total force in dynes with which 

 the armature is pulled by one field core is 



