MAGXK'riC DKSKiX ()K KKLAYS 3!) 



lo nia,u;iu't ic iron in tlic I'cccnlly (Knclopcd wire spring •fciid'al pui'posc 

 i-('lay. 



Magnetic iioii is a \ei'y low caihon steel of liigii purity. ( 'oniinei'cial 

 mild steel, properly annealed, will serve as an adequate suhsiiiute wlieie 

 tlu^ spread in properties shown in Table I ran be tolerated. 



In magnet design, casting offers the possibility of providing a more 

 complex one-piece configuration than can be obtained with punched 

 and formed parts. Casting is rarely used in current magnet const I'uct ion 

 — an illustrative exception is the British Post Office stepping magnet 

 for theii' step-by-step switch. (Irey cast iron has been a preferr(>d ma- 

 terial for this type of construction. 



The properties of nickel are included because of its use as a magnetic 

 separator and hinge member. 



Hyperbolic Approximation to Magnetization Curves 



The variation of permeal)ility with density makes it necessary to 

 provide some formulation of the n versus B relation in (le^'eloping an 

 analytical treatment of magnetization relations. The B versus // relation 

 for decreasing magnetization, the loop 2-3-4 of Fig. 4, has a shape similar 

 to that of a rectangular hyperbola, asymptotic to the line repi'esenting 

 B". This curve can therefore be represented approximately by the 

 eciuation : 



B 



= ^(Hc- H), (10) 



B" - B B 



in which ju", B'\ and He are constants. 



This purely empirical relation is called the Froelich-Kennelly equation. 

 In general, it does not give a satisfactory fit to the whole loop, but pro- 

 vides a satisfactory approximation for engineering use to the portions of 

 the curve in either the first or second quadrants, using different values 

 for the constants in the two cases. Tn addition, it may be employed to 

 represent the upper portion of the normal, or increasing magnetization 

 curve. 



The expression for the permeability ^u, or B/{Hc + H) corresponding 

 to (10) is: 



l^^^ Hc + H 



MM Li 



Hence, to the extent the B — H curve conforms to equation (10), the 

 reciprocal of the permeability varies linearly with H. Alternately, the 



