390 



BELL SYSTEM TECHNICAL JOURNAL 



less permanent shift of permeability due to accidental strong magneti- 

 zation. Such variations have recently been overcome to a degree in 

 continuous cores made of hard rolled nickel-iron alloy sheet,® but they 

 have been found to be rather large immediately after strong magnetiza- 

 tion, decreasing slowly to tolerable limits only after two or three days. 

 With compressed powdered molybdenum-permalloy cores, permea- 

 bility shift due to strong magnetization is remarkably small even within 

 a fraction of a minute after the magnetization is released, and any 



500 



450; 

 400] 



350 



300 



250 3 



200- 



50 z 



10 20 30 40 50 60 70 80 90 100 110 120 130 



CORE-FORMING PRESSURE IN TONS PER SQUARE INCH 



Fig. 3 — Eflfect of core forming pressure on density and tensile strength. 



further drift of permeability with time is negligible. In typical cores 

 of the new material, the shift in permeability after strong magnetiza- 

 tion is less than 0.2 per cent for cores of permeability 125, and less 

 than 0.05 per cent for cores of permeability 14. Figure 4 shows the 

 residual effect of the application and removal of various magnetizing 

 forces on cores of both these permeabilities. 



When a direct current is superposed on an alternating current in the 

 windings of a coil, the inductance is altered because the magnetic 

 field set up by the direct current modifies the core permeability. 

 Figure 5 shows the effect of superposed d-c. fields on the permeability 

 of 2-81 molybdenum-permalloy powder cores of various permeabilities. 



A further important core property is the constancy of permeability 

 with respect to flux density B. This is of particular importance in 



