LOADING FOR TELEPHONE CIRCUITS 233 



the side circuit coils each line winding is, in effect, distributed evenly 

 about the entire core. The necessary high degree of symmetry re- 

 quired by balance considerations is obtained by dividing each line 

 winding into two equal sections and interleaving them with the 

 sections of the other line winding; thus each complete line winding 

 consists of an inner section winding on one-half of the core and an 

 outer section winding on the opposite half core. Similar design 

 principles are applied to the phantom loading coils, with added com- 

 plications, however, arising from the increased number of line wind- 

 ings. Each of the four line windings consists of an inner section 

 winding located on one core quadrant and an outer section winding 

 located on the opposite core quadrant, the two line windings associ- 

 ated with a given side circuit being distributed about the same pair 

 of opposite core quadrants. In arranging the windings on the core, 

 precautions are taken to secure a symmetrical arrangement of the 

 direct admittances among the line windings and from the line windings 

 to the core and the case. 



It is interesting to note that the three-coil loading scheme illus- 

 trated in Fig. 4 was employed in the Boston-Neponset cable, installed 

 in 1910, which was the first successful installation of loaded phantom 

 circuits in the world. Other schemes of phantom group loading 

 using two-coil and four-coil arrangements have been developed here 

 and abroad, but none of them is considered to be as satisfactory as 

 the scheme illustrated in Fig. 4 from the standpoint of service and 

 cost. These other schemes are described in a recent article ^ which 

 compares them with the scheme above described. 



Loading Systems. In adapting the circuits to phantom working, 

 the electrical constants of the two-wire circuits were changed as little 

 as possible in making them suitable for use as side circuits of phan- 

 toms. In the cables, two pairs having different lengths of twist were 

 twisted into quad formation on a still different length of twist. The 

 required balance was obtained on open wire lines by cutting in a 

 large number of additional transpositions. 



The construction methods chosen resulted in the phantom circuits 

 having approximately 60 per cent, greater distributed capacity than 

 their side circuits, and a lower distributed inductance, approximately 

 in inverse proportion. It was obviously desirable to install the phan- 

 tom circuit coils at the same points as the side circuit coils; accord- 

 ingly, in working to the same standard of cut-off frequency, the rela- 

 tive circuit constants summarized above resulted in the phantom 



' "Commercial Loading of Telephone Cable," \V. Fondiller, Electrical Com- 

 munication, July 1925. 



