178 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



loading was substituted for H106 phantom loading, to provide HI 74-63 

 phantom-group loading as a successor standard for HI 74-106 loading. 



(b) E 174-106 Loading: The development work on HI 74-106 loading pre- 

 ceded that on the H44-25 system. By using available standard ''medium" 

 loading coils at standard "heavy" spacing, the transmission velocity and 

 the cut-off frequency were raised to values about 20% higher than those of 

 the H245-155 loading which had been, by far, the most widely used loading 

 on 19 and 16 gauge toll cables. The new combination of inductances and 

 spacings became widely known in the early installations as "medium-heavy, 

 high cut-off" loading. This designation called attention to the first change in 

 standard loading cut-off frequency since the establishment of the initial load- 

 ing standards in 1904. 



When used in conjunction with improved repeaters, the HI 74-106 load- 

 ing enabled satisfactory transmission to be obtained over circuits about 

 twice the length of the longest H245-155 repeatered circuits which were 

 satisfactory from the transmission standpoint. In the beginning, HI 74-106 

 loading was extensively used on 4-wire repeatered circuits. After the new 

 transmission systems using H44-25 loading came into general use, HI 74-106 

 (and HI 74-63) loading was largely restricted to short haul two-wire circuits. 



The 1917 trial installation tests showed H174-106 loading to be a substan- 

 tial step forward in the struggle to extend the transmission range in re- 

 peatered 19 and 16-gauge toll cables, but far from a big enough step to 

 satisfy the transmission requirements in very long cables such as the New 

 York-Chicago cable project which had been accepted as a definite develop- 

 ment objective. The continuing studies which considered other combina- 

 nations of lower inductances and of standard and new spacings ended in 

 the decision to standardize H44-25 toll cable loading. 



(c) H44-25 Loading: Although these inductance values had been used for 

 impedance matching loading on entrance cables, they had not previously 

 been used on toll cables. The initial designation for the improved loading 

 was "extra-light, very high cut-off " loading. The cut-off frequency and 

 transmission velocity were about twice as high as those in HI 74- 106 loading, 

 and the nominal impedance was 50% lower. 



H44-25 was necessary for the longest repeatered circuits. It was developed 

 primarily for use on 19-gauge 4-wire circuits in which large repeater gains 

 could be obtained by the repeaters in the one-way paths, to offset the rela- 

 tively high bare-line attenuation. The first installation of H44-25 loading 

 was made during 1919 on circuits in the New York-Philadelphia-Reading 

 Cable. Trial service of a complete four-wire system, including new regulating 

 and equalizing arrangements, and an echo suppressor, started during 1923. 

 In October 1925, commercial service between New York and Chicago started 

 over a H-44-25 four-wire repeatered system. 



It had taken a long time to work out the necessary improvements in the 



