388 BELL SYSTEM TECHNICAL JOURNAL 



phantom loading unit, in a 3-compartment case. This was the beginning of 

 the use of "phantom loading units" in open-wire loading. To provide 

 installation flexibility, cases were also developed for individual side and 

 phantom loading coils. The over-all dimensions for all cases were small 

 enough to avoid limitations on the number of circuits that could be loaded 

 at the same loading points along any line. Double-pole H-fixtures, how- 

 ever, were required on routes having a large number of wires. 



These air-gap type loading coils and their cases remained standard for 

 open-wire loading until subsequent developments in the art, notably im- 

 provements in the repeater and the use of open-wire carrier systems, re- 

 sulted in the gradual abandonment of open-wire loading. 



Following the transcontinental project, wire-core coils with air-gaps 

 were also developed for use on coarse-gauge duplex cables of the Boston- 

 Washington type. It is also of interest to note that while the work on the 

 transcontinental project was still under way a very good start was made on 

 the development of the compressed magnetic powder core-t}^e loading coil 

 for smaU-gauge cables. The high stability characteristics of this general 

 type of coil became an important factor in the wide use of telephone repeaters 

 in the long distance cable plant. 



Spacing Irregularities: Taking up the consideration of the impedance 

 irregularities caused by loading spacing irregularities, the need for a sub- 

 stantial improvement was duly proved, and precision limits of ±2 per cent 

 in the spacing were established for Unes to be used with high-gain repeaters, 

 starting with the transcontinental project. In general, the required pre- 

 cision in new lines could be secured by proper engineering care, including 

 more uniform transposition la3'outs, since coordination with the coil spacing 

 was necessary. In applying the new high stability coils along old routes, as 

 was necessary on the transcontinental line sections east of Denver, reloca- 

 tion of many of the loading points was found to be desirable. In these 

 loading rearrangements and sometimes also on new Hues, it was occasionally 

 found to be desirable to tolerate the use of geographically underlength 

 loading sections and build out their total capacitance to the theoretically 

 desirable values by using shunt condensers. Mica condensers having suit- 

 able dielectric strength were used for this purpose, protected by loading coil 

 t\'pe lightning arresters. In lines used for phantom working, a network of 

 six condensers was used to provide the optimum building-out capacitance 

 for the side circuits and their associated phantom. Subsequently, building- 

 out condensers and stub cables also found use in the repeatered loaded cable 

 plant for correction of objectionable spacing deficiencies. 



Incidental Cables: In improving the open-wire lines for repeater operation, 

 substantial development and engineermg effort was also devoted to the 

 reduction of impedance irregularities caused by incidental cables. Es- 



