414 BELL SYSTEM TECHNICAL JOURNAL 



With this source of unbalance between the main core and the sea 

 earth core removed or greatly reduced, it becomes increasingly im- 

 portant that the factors affecting the pick-up and the transmission of 

 interference on the two cores be made as nearly as possible the same. 

 In manufacturing the cable, core lengths should be paired off in such a 

 manner that the electrical constants of any portion of the sea earth 

 core match the constants of the corresponding portion of the main 

 core, and the two cores should preferably be armored together. 



By an extension of the method employed in deriving formula (5) an 

 expression for the interference-susceptibility frequency characteristic 

 of a cable having a balanced type of sea earth can be derived. This 

 expression will consist of two terms, the first representing the resultant 

 interference due to lack of balance between the sea earth conductor 

 and the main core, and a second term, similar in form to (5), repre- 

 senting the interference picked up on the portion of the cable beyond 

 the sea earth termination. Because of the difficulties involved in 

 balancing, there is a value below which the first term cannot practically 

 be reduced, which residue amounts to a few per cent of the magnitude 

 of interference that would be encountered on this portion of the cable 

 if the balanced type of sea earth were not employed. The second term 

 can be reduced to any desired value by terminating the sea earth in 

 water of sufficient depth. It is evident that when the sea earth has 

 been extended to a point where the second term is small compared 

 with the first, the limit of interference reduction is reached. 



The question as to how far from shore the sea earth should be located 

 in a particular case is an economic problem, the optimum location 

 being that where the increase in value of the cable, due to diminution 

 of interference by further extension of the sea earth, balances the ad- 

 ditional cost of making the extension. In some cases it is found eco- 

 nomical to obtain the desired ratio of signal-to-interference by means of 

 a more efficient and expensive core rather than by an extended sea 

 earth conductor. In the case of transatlantic cables terminated at 

 points on the English Channel, or on the North Sea, for example, sea 

 earth conductors several hundred miles in length are required in order 

 to get a deep water termination. By increasing the weight of the main 

 conductor, thereby increasing the amplitude of signals received over 

 the cable, a greater amount of interference can be tolerated, in which 

 case a comparatively short sea earth can be employed, just long enough 

 to get rid of local interference and of the pick-up of signals from cables 

 terminating nearby. 



An inductively loaded submarine telegraph cable possesses charac- 

 teristics which make the balanced type of sea earth particularly 



