HIGH-SPEED OCEAN CABLE TELEGRAPHY 265 



The cost and physical characteristics of insulating materials are, 

 of course, factors of great importance. With few exceptions gutta- 

 percha or compounds consisting mostly of gutta-percha have been 

 used for long submarine cables. The cost of the gutta-percha insula- 

 tion is a large part of the whole cost of a cable and the fact that its 

 cost is high leads to using the least amount consistent with maintaining 

 safe insulation. If a very much cheaper substitute or one of superior 

 electrical properties were available, the basis of design of loaded deep- 

 sea cables might be somewhat changed. 



Even the sheath of armor wires is capable of considerable improve- 

 ment as regards its effect on the behavior of a loaded cable. As 

 pointed out previously, the sheath introduces electrical resistance due 

 to the fact that it carries some of the return current which at high 

 frequencies tends to concentrate around the cable. Armor wire of 

 higher resistivity would introduce less resistance in this way or an 

 electrical improvement might be obtained by consideration of this 

 effect in the mechanical design of the cable sheath. At very high 

 frequencies where the return current is closely concentrated around 

 the cable the armor wire has an opposite effect and an improvement 

 can be obtained by decreasing its resistance or by the addition of 

 other conductors in parallel with it as was done on the Key West- 

 Havana telephone cables. Some electrical resistance is introduced by 

 the magnetic coupling between the sheath and the conductor due to 

 the helical shape of the loading material and the armor wires. This 

 effect is small in the cables which have been made but might be large 

 in higher speed cables and should be taken into account in the design 

 of such cables. A similar effect results from the use of the ordinary 

 teredo tape on loaded cables. 



With improvements in materials and construction which permit 

 higher operating speeds and with the demand for more efficient means 

 of handling a large volume of cable traffic, greater importance will 

 doubtless be attached to duplex or simultaneous two-way operation, 

 and it is of interest to consider some of the ways in which duplex 

 operation might be secured. Of course there is no reason to assume 

 that the loaded cables which have already been laid will not eventually 

 be operated duplex although they were designed primarily with 

 simplex operation in view. From the studies of both types of opera- 

 tion which we have made it appears more economical for the present 

 to operate the existing transatlantic loaded cables one way at a time. 

 Indeed this type of operation with automatic reversing apparatus 

 possesses many advantages over the ordinary duplex methods applied 

 to non-loaded cables. If, however, a loaded cable were designed 



