748 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



similar to those used with similar-sized toll cable coils. The potting com- 

 plements for incidental cables, however, were small relative to the com- 

 plements most generally used in the toll cables. Occasionally, in situations 

 where toll entrance facilities and long-distance cable facilities shared the 

 same cable for a short distance, the potting complements would include both 

 types of loading. The color code used on the coil terminal quads in the stub 

 cables of the loading coil cases facilitated identification of the different types 

 of loading in the cable splicing-operations. 



(V-B) Carrier Loading for Incidental Cables in Open-Wire 

 Carrier Systems 

 Historical 



The first open-wire carrier telephone system was installed late in 1918, 

 and in the early 1920's general commercial use began to expand rapidly. A 

 comprehensive account of the pioneering development work is given in a 

 1921 A.LE.E. paper^^ by E. H. Colpitts and O. B. Blackwell. 



Experimental types of carrier loading were made available for use on 

 incidental cables in the open-wire lines on which the first carrier systems 

 were installed. In general, these early carrier loading installations were 

 engineered to specific job requirements. 



C4.1 and C4.8 Loading: From this experience there evolved a quasi- 

 standard loading treatment which served the current service needs, pending 

 completion of the development of the first standard carrier loading systems, 

 C4.1 and C4.8, late in 1923. These were designed to provide good impedance- 

 matching up to a top frequency of about 30 kc. During the intervening years 

 this loading has remained standard for incidental cables in carrier systems 

 using this frequency-band, even though important changes have been made 

 in the carrier systems themselves, notably the first Type C carrier systems^^ 

 during the middle 1920's, and the improved Type C systems^^ during the 

 late 1930's. 



B15 Loading: During the late 1920's a lower cut-off carrier loading system 

 designated B15 was designed especially for use with carrier facilities operat- 

 ing below a top frequency of about 10 kc. This loading served a double 

 purpose. It was suitable for use with the old standard Type B carrier 

 telegraph system'^ and with the new standard, single-channel, Type D 

 carrier telephone system.^^ (In many of its early applications the Type B 

 telegraph system used the frequency space between the voice circuit and the 

 carrier telephone channels.) The B15 loading system is still in good standing. 

 When an improved single-channel telephone system, Type H^^ was developed 

 during the late 1930's, its frequency allocation was chosen so that it could 

 use "spare" B15 circuits which had become available on a substantial 

 mileage of incidental cables. 



