388 BELL SYSTEM TECHNICAL JOURNAL 



light (0.5 micron). This thickness of insulating film may be shown to 



rt 

 be approximately - ^^-^ , where r is the percentage of insulating ma- 

 6\J\)p 



terial by volume, p is the packing factor of the magnetic material, and 



/ is the r.m.s. particle diameter (assuming spherical particles). 



A new type of ceramic insulation has been introduced with these 

 cores which fulfills the above requirements and which is more inert 

 than the previous type. This new insulating material is free from 

 water soluble residue. It thus eliminates the final washing treatment 

 which was required with the earlier type. 



For applications where a low permeability is desired, non-magnetic 

 powder is added to further dilute the magnetic material. The permea- 

 bility of the finished core depends largely on the quantity, particle 

 size, and thoroughness of admixture of non-magnetic powder. Vari- 

 ous attempts to derive theoretical relations between core permeability 

 and dilution have been made,"'^^ but they generally fail in some 

 detail. An empirical representation of this relationship is found to be 



H = niP or log fi = p log Hi, 



where m is the intrinsic permeability of the magnetic material, and p 

 is the packing factor, or fraction of the core volume occupied by mag- 

 netic material. This equation is found to be valid for a wide range of 

 dilution, effected either by adding insulating material or by reducing 

 the load during core compression. However, the intrinsic permeability 

 must be determined experimentally for each type of particle, size 

 distribution, method of admixture of non-magnetic powder, and an- 

 nealing process. Figure 2 shows curves of permeability and percentage 

 diluting material vs. metallic packing factor. A permeability of 125 

 has been selected for most loading coil cores, while permeabilities of 14 

 and 26 have been chosen for two important types of high-frequency 

 filter coils. 



A pressure of 100 tons/sq. in. is employed in forming molybdenum 

 permalloy cores, to attain proper density and mechanical strength. 

 The elTect of pressure on core density and strength is shown in Fig. 3 

 for cores having 2.5 per cent dilution. The tensile strength of diluted 

 cores is decreased somewhat ; for example, cores with 25 per cent non- 

 magnetic materials have a tensile strength of about 250 Ibs./sq. in. 



In annealing the compressed cores to remove stresses incident to 

 pressing, it has been found that the insulating material remains intact 

 at a considerably higher temperature if oxygen is excluded. An im- 

 proved annealing treatment has therefore been introduced by which 



