HIGH-SPEED OCEAN CABLE TELEGRAPHY 261 



to within less than 5 per cent with the values computed as described. 

 Part of this difference is probably due to the fact that the computed 

 values of sea-return resistance are smaller than those actually en- 

 countered on the cable. A possible explanation of this effect is that 

 the electrical resistivity of the earth beneath the cable is considerably 

 higher than was assumed in the theoretical development of the formula 

 for sea-return resistance. 



A General Survey 



The preceding discussion has referred principally to the progress in 

 certain lines of development which were chosen as best suited to 

 accomplish the result of high-speed ocean cable telegraphy. It is of 

 interest now to consider in a general way the field of application of 

 loaded telegraph cables and the nature of modifications which might 

 be made in their construction and operation. 



All of the loaded telegraph cables to which I have referred are 

 relatively long. That permalloy loading should have been applied 

 first to long cables is the natural consequence of the facts that the 

 need for increased cable speed was principally between points far 

 apart and that the greatest economic gain from loading could be 

 obtained with long cables. Where high-speed cable operation was 

 desired between points only a few hundred miles apart, it could readily 

 be obtained with a non-loaded cable by merely making the cable 

 large enough to give the required speed. Accordingly for short 

 cables the operating speed was determined either by the demand for 

 communication or by limitations of terminal apparatus. But where 

 the necessary length was of the order of 2000 miles or more, even the 

 heaviest cables which were considered practicable to lay and maintain 

 were limited by the inherent characteristics of non-loaded cables to 

 relatively low speeds, and it was accordingly for such great distances 

 that the manifold speed advantage of the permalloy-loaded cable was 

 of the greatest value. 



To give a fair numerical estimate of the advantage of loading long 

 cables is extremely difficult since the result depends so much on the 

 basis of comparison and the limitations of size and operating require- 

 ments which are imposed. Probably the most nearly fair basis of 

 comparison would be the relation of cost to speed for the old and for 

 the new cables, but to make such a comparison requires data on cost 

 which is forever changing. An interesting basis of comparison from 

 the technical point of view is the ratio of the traffic capacity of a non- 

 loaded cable operated duplex to that of a loaded cable operated 



