MAGNETIC MATERIALS IN TELEPHONE SYSTEM 461 



mechanical workability, and low cost are desirable for such appli- 

 cations. 



5. Magnetostriction 



The relative change in length of a magnetic bar upon magnetization, 

 bljl, ranges from negative to positive values, depending on alloy com- 

 position, and is roughly proportional to B". If a polarizing field is 

 applied, small additional alternations of field will give accompanying 

 and nearly proportional alternations of length of a bar. These alterna- 

 tions evidence themselves in the electrical constants of a coil enclosing 

 the bar. 



This efifect can be utilized in oscillators and filters for frequencies 

 which involve the use of mechanically resonating bars of convenient 

 size. Among the high permeability materials, 45 permalloy appears 

 to give the largest magnetostrictional effect.'*^ In order to limit eddy 

 current losses, the material must be laminated more or less finely, 

 depending on the frequency. 



The inverse magnetostriction effect by which e.m.f. is generated in 

 a coil when the core is vibrated, becomes objectionable as a source of 

 circuit noise in the transformers of high gain amplifiers.'^ An alloy 

 with minimum magnetostriction such as 81 permalloy or 4r-79 Mo- 

 permalloy is preferred for such cases. 



Other efTects of magnetostriction are the generation of sound by 

 the cores of coils subject to alternating magnetization, and the appear- 

 ance of undesired resonance effects in the electrical circuit at fre- 

 quencies at which the core resonates mechanically. 



6. Thermal Variation of Permeability 



The initial permeability of ordinary magnetic materials increases 

 more or less slowly with increasing temperature, until a maximum 

 value is reached, above which temperature the permeability declines 

 very rapidly to the non-magnetic, or Curie point. The Curie point of 

 an alloy can be moved down the temperature scale by adding non- 

 magnetic materials, such as Mo, Cr, Cu, etc., to the alloy. ^^ 



The positive temperature coefficient of inductance of powder core 

 coils due to thermal change of permeability becomes objectionable in 

 crystal filters, where only very small variations in the resonant fre- 

 quencies can be tolerated.*' It is made slightly negative to counteract 

 the small positive coefficient of mica condensers by the admixture with 



« A. Schulze, Zeit. f. Phys. 50, 448 (1928). 



«2 G. W. Elmen, Bell Labs. Record 10, 2 (1931). 



« C. E. Lane, B. S. T. J. 17, 125 (1938). 



