as 10,000 miles a second. With the greater distances for which cable 

 circuits of the four-wire type could be used, it was found that the time 

 of transmission required at a velocity of 10,000 miles a second was 

 great enough to introduce additional difficulties in the provision of 

 satisfactory transmission. The nature of these difficulties and of the 

 means adopted for overcoming them will be discussed a little later. 

 However, it must be mentioned here that to overcome these difficulties, 

 it was necessary, for these longer circuits, to devise new loading 

 systems which provided circuits with a velocity of 20,000 miles a second 

 and at the same time had the advantage of transmitting a broader 

 band of frequencies, although they had the disadvantage that the 

 circuit had higher transmission losses per mile and therefore required 

 greater amounts of amplification. These higher velocity circuits are 

 in general use for all long cable circuits and have been found satis- 

 factory up to the greatest distances spanned by cables in this country 

 at the present time, namely, approximately 2500 miles. 



Cables are placed either underground or supported overhead from a 

 steel messenger strand strung on poles. At the present time approxi- 

 mately 47 per cent, is overhead and 53 per cent, underground. For 

 the most part the underground cable is pulled into permanent under- 

 ground conduit of vitrified clay. Some use has been made, however, 

 of cable buried directly in the ground, the lead sheath being protected 

 either by layers of jute impregnated with asphaltum compounds or by 

 a combination of such layers of jute and wrappings of steel tape. A 

 small use has also been made of a single duct made of compressed fibre 

 for the protection of underground cables. 



The conductors used for long telephone circuits are quadded for 

 phantom operation and are largely of 19 A.W.G. although some use 

 has been made of 16 A.W.G. for the shorter circuits because of a 

 possible saving in the numbers of repeaters with the larger gauge in 

 those cases. Many of the cables include a number of special 16- 

 gauge pairs provided specifically for program transmission circuits 

 and equipped with loading and amplifiers designed particularly for 

 that form of service. Figure 15 shows schematically the arrangement 

 of conductors of a standard type of full size cable (outside diameter 

 2^^ inches) which is in common use. 



With these developments and other auxiliary developments which 

 will be discussed later toll cables have come to have a very important 

 place in the provision of toll telephone service by the Bell System. 

 The percentage of toll wire in cable has increased from 30 per cent, in 

 1915 to 82 per cent, at the present time. The present toll cable net- 



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