LOADED TELEGRAPH CABLES 401 



two different media is a problem of considerable difficulty. An 

 approximate method, which gives results which are sufficiently accurate 

 for purposes of cable design, consists in computing the combined 

 impedance of the three parallel conductors, namely, the armor wires, 

 the sea water, and the earth, the impedances of the latter two con- 

 ductors being determined by the methods outlined in the afore- 

 mentioned paper. The physical interpretation of the sea return 

 resistance as obtained by this method is that the high value of re- 

 actance of the sea water and earth, due to the large cross-section of 

 the conducting area, forces the return current to flow in the armor 

 wires even though the resistance of this path is much higher than 

 that of the paths through the sea water and earth. It appears probable 

 that the electrical conductivity of the earth is very much less than 

 that of sea water which would result in a larger cross-section of con- 

 ducting area external to the armor wires and larger inductance of this 

 path. This leads to higher values of sea return resistance than are 

 obtained on the assumption that the cable is surrounded on all sides 

 by sea water and thus gives a result more nearly consistent with the 

 observed facts. 



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