184 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



important transmission-performance characteristics, relative to those of the 

 standard powdered-iron core loading coils. 



9.2 Permalloy Core Loading Coils 



The coils standardized for toll cable loading were in volume and weight 

 about one- third as large as the superseded types previously described. Coils 

 C and B in the headpiece exemplify the coils under comparison. The direct- 

 current resistances of the new coils were slightly lower than those of the 

 superseded designs, their hysteresis losses were substantially lower, but their 

 eddy-current losses were somewhat greater, because of the higher permea- 

 bility. In consequence, their effective resistances were more favorable at the 

 important low and middle speech-frequencies, than those of the compressed 

 annealed iron powder core coils but not quite as good at the top frequencies. 



The stability of residual inductance after magnetization by strong super- 

 posed currents was appreciably better than that of the superseded designs. 

 The hysteresis advantages included a substantial reduction in non-linear 

 distortion effects and about a 50% reduction in the transient distortion 

 effects that are unavoidable in the operation of grounded telegraph over 

 composited circuits. Their telegraph-flutter rating was considerably better 

 than that of any of the prior standard cable loading coils, excepting only the 

 large-size "high-stability" type of coarse-gauge cable coils, previously de- 

 scribed. (Subdivision 5.) 



With the standardization of the permalloy-core loading coils there started 

 the practice of coding the combination of two side circuit coils and the 

 associated phantom coil as a phantom-group ''loading unit." The letter 

 "P" was used in the code designations; a code number associated with the 

 code letter recognized the different inductances of the different loading 

 units, the complete codes being PI, P2, etc. 



The coil size-reduction resulted in a 2 to 1 reduction in the potting-space 

 requirements and permitted twice as many coils to be potted in standard- 

 size cases, thus reducing potting costs. The cost savings in potted coils 

 ranged from 30 to 40%. Additional savings resulted in the installation costs, 

 including the space costs in the loading vaults in underground cable projects. 



The development was very timely, in that the much cheaper coils became 

 available during 1928 for use in the unprecedented expansion of H44-25 

 four-wire repeatered facilities that started that year and built up to a very 

 high peak during 1930. In the period 1928-1931, over 4,000,000 permalloy- 

 core toll cable loading coils were manufactured for Bell System uses. The 

 lower costs of these coils encouraged the provision of larger circuit-groups in 

 the long-distance cable plant which made possible substantial improvements 

 in the speed of service. This, in combination with excellent transmission 



