448 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



in the toll plant is quite expensive in relation to the feasible gains in the 

 exchange plant. Consequently, there has not been an extensive use of re- 

 peaters in the exchange plant. Looking towards the future, however, the use 

 of a low-cost telephone repeater of an entirely new design (Type El) is ex- 

 pected to result in a much more extensive use of repeaters, and in conse- 

 quence some considerable reduction in the demand for the heaviest weights 

 of exchange area loading. 



(15) First Two Decades of Commercial Loading 



15.1 General 



For about two decades after the estabUshment of the first standard cable 

 loading systems (Table II, page 156), medium-weight loading was by far the 

 most extensively used standard on exchange cables. However, a few of the 

 longest exchange cables used heavy-weight loading. Also in some areas 

 there was a moderate use of light-weight loading on short cables. 



In the period under discussion almost all of the exchange area loading 

 was installed on 19 ga. cables in situations where, without loading, the 

 circuit lengths and the transmission requirements would have forced the 

 use of much more expensive 13 ga. or 16 ga. cables. Twenty-two gauge cable 

 was available for subscriber cables and for short inter-office trunks. Nine- 

 teen gauge non-loaded cable was used on short inter-office trunk cables, 

 however, as it was then more economical than loaded 22 ga. cable, and had 

 a greater supervision and signahng range. In the larger metropoHtan areas, 

 loading was much more generally used on trunks to tandem-switching office 

 and on connecting-trunks between local and toll offices, than on the direct 

 inter-office trunks, because of the much more severe transmission limits 

 imposed on the tandem and toll office trunks. In occasional instances, these 

 requirements made it necessary to use loaded 16-ga. circuits. There was 

 also a large use of loading on trunk cables between city tandem offices and 

 suburban local offices. By avoiding the need for 13-ga. cable and by greatly 

 reducing the need for 16-ga. cable in these important fields of use, the 

 introduction of loading made possible very large savings in the first costs 

 of additions to the rapidly expanding new plant, and in the subsequent 

 annual charges. 



15.2 Partial Loading 



In the course of the expansion of exchange area loading a practice of 

 "partial loading" evolved. This is exemplified by the loading of a part of a 

 trunk circuit when it exceeds by a moderate amount the length that would 

 be satisfactory from the transmission standpoint without loading, instead 



