88 



BELL SYSTEM TECHNICAL JOURNAL 



Mo 



40 45 50 56 60 65 70 75 80 85 90 95 100 



COMPOSITION OF PERMALLOY — PEP CENT NICKEL 



Fig. 25 — Comparison of the theoretical upper limit of initial permeability (Ker- 

 sten) with the highest initial permeabilities observed for iron-nickel alloys (Arnold 

 and Elmen, Schulze). 



The efifects of strain will now be summarized briefly. The origin 

 of the efi'ects Hes in the magnetic action between neighboring atoms. 

 The magnetic action is balanced by the elastic (electrostatic) forces 

 between atoms. The balance of these forces results in a change in 

 shape of the magnetic body when it is magnetized (magnetostriction), 

 and also a change in magnetization resulting from strain (strain- 

 sensitivity). Magnetization may be either aided or hindered by a 

 homogeneous uniaxial strain, the effect depending on the mag- 

 netostriction in a way that can be estimated qualitatively but not 

 quantitatively. But material in which local strains are directed at 

 random is more difficult to magnetize because the strains prevent a 

 change in magnetization; and the more intense such strains are, the 

 harder the material is to magnetize or demagnetize. The effect of local 

 strains upon the initial permeability can be calculated with fair success, 

 but other magnetic quantities, such as maximum permeability, can 

 as yet be estimated in a qualitative way only. 



