154 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



came standard for 165-mil lines in 1910, after a long period of additional 

 development work to get better line-insulation. 



All of this early loading development work was for non-quadded cables 

 and for non-phantomed open-wire lines. 



Loading Coils 



The earliest speculative suggestions regarding coil loading recognized 

 the critical need for obtaining a low ratio of coil resistance to circuit re- 

 sistance, and by implication a low ratio of coil resistance to coil inductance. 

 As was expected, this turned out to be a difficult design and manufacturing 

 problem, especially with open-wire loading which was given development 

 priority. 



By April 1901, a very satisfactory coil-design had been worked out for 

 open-wire loading by Mr. H. S. Warren, an associate of Dr. G. A. Campbell. 

 It had a toroidal core, formed by winding a bundle of insulated mild-steel 

 wires, 4 mils in diameter, on a suitably shaped spool, several miles of wire 

 being used in each core. The manufacturing process of the outside supplier 



Fig. 1 — Non-phantom type loading coils. Coil winding schematic and method of con- 

 nection into circuit. 



included cold drawing to obtain a magnetically hard wire having an initial 

 permeability of about 65. The two line-windings, each confined to separate 

 halves of the core winding-space, made use of insulated, stranded wire. The 

 fine subdivision of the magnetic material and of the copper conductor was 

 essential to the satisfactory control of eddy current losses. 



This coil. Code No. 501, was the first standard loading coil. It remained 

 standard for about a decade, until a redesign became necessary to facilitate 

 an extensive commercial exploitation of phantom working. Because of the 

 very low-resistance design objective, it had to be a large coil. Some of its 

 dimensional and electrical characteristics are included in Table I. In size, 

 it was approximately 25% larger than the largest coil shown in the head- 

 piece. 



The cable loading coils listed in the table were standardized for general 

 use during 1904, following occasional use of other types of coils having 

 different inductances and in some instances using a different core material. 

 These coils were generally similar in their basic design features to the open- 

 wire loading coil, but for economic reasons were much smaller in size.^*'^ 



^•^ Core weight 3.5 lbs., 69000 turns iron wire; length 11 miles. 



