236 BELL SYSTEM TECHNICAL JOURNAL 



insulation shall be maintained and that the core, which comprises 

 the conductor and insulation, shall not be damaged in laying or in 

 subsequent repairing operations. Electrically, the cable must be so 

 designed that it will serve properly to transmit signals. Thus the 

 electrical design is concerned with the size of the conductor and the 

 amount and characteristics of the loading material and insulation, 

 whereas the mechanical design is concerned with the mechanical 

 characteristics of the conductor and its insulation and with the jute 

 and armor wire which serve to protect the core and give the cable the 

 necessary strength. These two aspects of design cannot, of course, 

 be considered quite independently of each other and both are in the 

 ultimate analysis controlled by economic considerations. It is con- 

 venient for our present purposes, however, to consider them separately. 



The mechanical design of cables is a well-established art and so 

 great are the difficulties of laying and maintaining cables, even under 

 the most favorable conditions, that it is desirable to avoid taking any 

 liberties with this phase of cable construction. Fortunately the 

 method of loading by a continuous wrapping of magnetic tape or 

 wire introduces no need for radical change in the important mechanical 

 features of the cable. 



The copper conductor of the loaded cable is, as in many non-loaded 

 cables, composed of a central copper wire surrounded by several flat 

 copper strips. This form of conductor is flexible and economical of 

 space, and the fact that it has several strands reduces the chance of a 

 complete break. The loading tape or wire furnishes additional 

 protection in this regard. 



The thickness of gutta-percha must be sufficient to insure the 

 integrity of the insulation at all points. It is, in fact, this consideration 

 which established the amount of insulation used on the loaded cables 

 which have been laid, since consideration of the theoretical economic 

 optimum thickness of gutta-percha would in each case have demanded 

 less gutta-percha than is considered safe. In this regard the insulating 

 problem of the loaded cable is like that of the non-loaded cable. 



The disposition of jute and armor wire around the core is determined 

 wholly by mechanical considerations as in the case of the non-loaded 

 cable for which the practice is fairly well standardized. Unlike the 

 non-loaded cable, however, the loaded cable, in its electrical behavior, 

 is affected somewhat by the presence and character of the armor wire 

 as will be described later. 



As is well known, the electrical behavior of non-loaded cables is 

 determined almost wholly by their resistance and capacity and conse- 

 quently the only important features to consider from the electrical 



