PRESENT STATUS OF FERROMAGNETIC THEORY 



SI 



65 



Sq- 



Otr 



§2 



Fig. 



40 45 50 55 60 65 70 75 80 85 90 95 100 



COMPOSITION OF PERMALLOY — PER CENT NICKEL 



16— Saturation magnetostriction of the permalloys (McKeehan and Cioffi, 



Schulze). 



definite magnetic moment as a result of the spin and orbital motion of 

 the electrons. This supposition makes it possible to calculate the 

 magnitude of the mutual magnetic forces which are opposed by the 

 elastic forces holding the crystal together. For iron, the calculations ^ 

 indicate that equilibrium is reached when there has been a slight 

 increase in length in the direction of magnetization and a decrease in 

 length at right angles to this direction such that the volume remains 

 practically unchanged. This calculated magnetostriction is in agree- 

 ment with experiment as to sign and order of magnitude. With 

 nickel the agreement is not so satisfactory. But in each case the 

 theory is clear in predicting the proper qualitative relationship between 

 magnetostriction and change in magnetization caused by strain. 



Fig. 17 — The magnetic forces between atoms cause a slight elongation in iron 



(magnetostriction) . 



