INDUCTIVE LOADING FOR TELEPHONE FACILITIES 



155 



Also, their ratios of resistance to inductance were considerably higher. In 

 size, these coils are typified by Coil B in the headpiece. 



The 4-mil (No. 38 A.W.G.) wire used in these cable coil cores had a 

 nominal initial permeability of 95, nearly 50% higher than that of the 

 open-wire loading coil core-material. The differences in magnetic perform- 

 ance characteristics, including permeability, core losses, and certain other 

 features subsequently discussed, resulted solely from differences in the 

 annealing treatments during the wire-drawing process. 



Additional important differences between the cable coils and the open- 

 wire coil were: (1) the use of non-stranded conductors in the windings of 

 the cable coils, and (2) the use of lower dielectric-strength insulation in 

 the cable coils. 



Table I 

 Characteristics of First Standard Loading Coils 



Loading Coil Cases 



The open-wire loading coils were individually potted in cast-iron rases 

 designed for mounting on pole fixtures. The coil terminals issued from the 

 case in individual rubber-insulated leads. 



The cable loading coils were assembled in multi-coil groups on wooden 

 spindles, and potted in cast-iron cases, suitable for installation in cable 

 manholes and on pole fixtures. Intercoil crosstalk was controlled: (1) by 

 mounting adjacent coils so there would be approximately a minimum 

 coupling between their (small) magnetic-leakage fields; (2) by using iron 

 shielding-washers between adjacent coils; and (3) by placing the spindle 

 groups of coils in individual compartments cast in the cases. The coil- 

 terminal leads issued from the cases in a twisted-pair, lead-covered, stub 

 cable. 



Prior to potting, the coils were thoroughly dried out under vacuum, and 

 were impregnated with moisture-proofing compound. 



Loading Systems 



As previously mdicated, the standard practices for cable loading were 

 established in advance of those for open-wire loading, notwithstanding an 



