1238 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



ratio of distributed inductance to distributed capacitance, incidentally 

 resulting in low impedances, and the relatively high conductor resistances 

 of cable circuits, gave loading its greatest opportunities in exercising its 

 natural functions of reducing the circuit attenuation and attenuation- 

 frequency distortion. Clearly appreciated from the beginning, these possi- 

 bilities have been advantageously realized to a very great extent, and they 

 still have substantial economic importance for future voice-frequency appli- 

 cations in the continuing growth of the exchange area non-quadded cable 

 plant, and short, quadded interurban toll cables. 



Open-Wire Loading 



The higher ratios of distributed inductance to distributed capacitance in 

 the open-wire lines made the reduction of attenuation-frequency distortion 

 a relatively minor objective in the use of loading, attenuation reduction 

 being the primary objective. Incidentally, the relatively high impedances 

 of the non-loaded lines that resulted from their higher ratios of inductance 

 to capacitance limited the attenuation reduction obtainable by coil loading 

 to smaller percentage values than those obtainable on cable circuits. How- 

 ever, full advantage of these important, though limited, possibilities was 

 realized in the expanding open-wire plant during the decade that preceded 

 the commercial introduction of vacuum-tube repeaters. The early uses of 

 these repeaters on open-wire lines were on circuits having improved loading 

 designed especially for use in conjunction with repeaters. In 1915, this combi- 

 nation of loading and repeaters made transcontinental telephony econom- 

 ically feasible, and for several years greatly increased the demand for 

 loading. The importance of open-wire loading soon started to decline, 

 however, as a result of improvements in the repeaters, their circuits, and 

 auxiliary networks, which made it possible to secure considerably better 

 voice-frequency transmission on long lines at a lower total cost by dis- 

 carding loading and using more repeaters. The climactic event in this new- 

 trend was the beginning of the operation of the first transcontinental 

 circuits on a non-loaded basis during 1920. During the middle and late 

 1920's the general removal of open-wire loading was expedited to increase 

 the plant flexibility and facilitate the commercial exploitation of carrier 

 telephone and telegraph systems over non-loaded lines. 



Since, for transmission-cost reasons, it is not feasible to develop suitable 

 loading for long lines over which carrier systems are operated, there is no 

 reason to expect any new leases of life for open-wire coil loading. Not- 

 withstanding its small extent of use relative to that for cable loading, and 

 the relatively short period during which it was standard practice, open-wire 

 loading was a necessary and a vitally important factor in the rapid expansion 

 of long-distance telephony that began nearly five decades ago. 



