350 BELL SYSTEM TECHNICAL JOURNAL 



of important new development projects, or the continuation of important 

 work which had been started at Boston. 



The first new major project was that of developing phantom group load- 

 ing for open-wire lines. The theoretical work on this problem started early 

 in October 1907, and the design requirements for the new types of loading 

 apparatus were put up to the Western Electric Company in December 1907. 

 The general objective was to make possible the exploitation of the econ- 

 omies inherent in a full application of phantom working in the expensive 

 open-wire plant. Other developments, mentioned later, were also essential 

 to the full achievement of this objective. The important fact to remember 

 in this general connection is that for a period of several years prior to the 

 development of phantom group loading it was feasible to load 104 mil 

 circuits, and to phantom non-loaded 104 mil circuits, but it was not possible 

 to combine the advantages of phantom working and loading. Two new 

 types of loading coils were necessary, one which became known as the side 

 circuit loading coil for use on the phantomed pairs, and the other for use 

 on the superposed phantom itself. The original standard open-wire load- 

 ing coils were not suitable for use on side circuits because of the transmission 

 impairments and the unbalances that they would have introduced into the 

 associated phantom circuits. The critical problem in the new loading 

 apparatus was to obtain satisfactorily low crosstalk among the associated 

 side and phantom circuits. The results of this development work are de- 

 scribed in a subsequent section. 



Among the important old projects that were continued and pushed in the. 

 months that followed the move from Boston were (1) studies and experi- 

 ments to enable loading to be used with satisfactory results on 165 mil 

 open-wire circuits, and (2) problems involved in the use of telephone re- 

 peaters on loaded lines. 



The problem of loading the 165 mil circuits was primarily one of improving 

 and stabilizing the insulation of the circuits, so that during wet weather and 

 the subsequent drying-out periods the transmission impairments caused- by 

 leakage losses would not materially offset the transmission loss reduction 

 obtainable with the added inductance. In the early commercial attempts 

 to load 165 mil circuits (beginning with the New York-Chicago line, 1901) 

 the loading eventually proved to have much too high an impedance in rela- 

 tion to the wet weather line insulation, and it was removed late in 1905 

 because under unfavorable weather conditions the transmission equivalent 

 became (temporarily) worse than that of a non-loaded 165 mil line. The 

 solution of the loading and insulation problems for the 165 mil lines required 

 a great deal more experimental work than had been involved in the success- 

 ful appHcation of loading to the 104 mil pairs, primarily because of the much 

 greater sensitivity of the heavier conductors to leakage effects. The open- 



