170 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



ferences were also involved. These two magnetically different, compressed 

 loading coil core-materials were sometimes known as ''soft-iron dust" and 

 ''hard-iron dust" or as "compressed annealed iron-powder" and "com- 

 pressed unannealed iron-powder," respectively. In the Speed-Elmen classic 

 paper^' on "Magnetic Properties of Compressed Powdered Iron," these two 

 materials are referred to as "Grade A" and "Grade B", respectively. A 

 still lower permeability core material, known as "Grade C", was developed 

 primarily for use in carrier frequency inductance coils. In this material, a 

 larger amount of particle insulation than that in the "A" and "B" grades 

 was used, and the average size of particle was smaller. 



In commercial production the "Grade B" cores consisted of a mixture of 

 90% unannealed powder and 10% annealed powder, the latter com- 

 ponent being included to obtain the desired value of permeability, and to 

 increase the mechanical strength of the core rings. 



The different magnetic characteristics of the "A" and "B" grades of 

 compressed, powdered iron were basic factors in the evolution of the loading 

 practices for the new loading coils that used them in their cores. A brief 

 review of these practice differences follows, and includes some additional 

 general data regarding the coils themselves. ^''^ 



6.2 Compressed, Annealed, Powdered-Iron Core Loading Coils 



Referring to Table III, it will be noted that cores using this new material 

 had nearly the same effective volume-permeability as the cores of 95- 

 permeability iron-wire. By using similar-size cores, and closely similar 

 windings, it was found possible to obtain effective resistance-frequency 

 characteristics close to those of the standard small-gauge cable loading coils 

 using 95-permeability wire cores, as typified in the 508 coils of Table I, 

 and corresponding grades of side circuit and phantom loading coils. Also 

 the new potting developments were minimized. 



An outstanding service advantage of the new "soft-iron dust" core 

 loading coils was in their very high stabiUty of residual inductance, by 

 virtue of the self-demagnetizing action of the very large number of very 

 small series air-gaps in the cores. After a temporary exposure to magnetiza- 

 tion by abnormally large superimposed currents that might be caused by 

 accidental grounds on superposed d-c signaling circuits or by induction from 

 outside sources (lightning, power-line shorts or grounds), the coil inductance 

 would return to within a few per cent of the initial value. On the other 

 hand, after extreme exposure to strong magnetic shocks the residual induct- 

 ance in the 95-permeability wire-core coils might be as much as 40% below 

 the initial inductance, the high retentivity of the magnetic circuit being 

 an important factor in this performance. 



*' Some additional detailed data regarding these two series of loading coils were pub- 

 lished in Reference (8). 



