366 HELL SYSTEM TECHNICAL JOURNAL 



around it, the degree of concentration increasing with the frequency. 

 With the return current thus concentrated the resistance of the sea 

 water is of considerable consequence. It is further augmented by a 

 resistance factor contributed by the cable sheath. This may be 

 better understood by considering the cable as a transformer of which 

 the contluctor is the primary and the armor wire and sea water are 

 each closed secondary circuits. Ob\iously the resistances of the 

 secondary circuits of armor wire and sea water enter into the primary 

 circuit and hence serve to increase the attenuation. Tiie presence 

 of the armor wires may thus be an actual detriment to the trans- 

 mission of signals. 



To take account of the hysteresis resistance, /?/,, and also of the 

 increased inductance and eddy current resistance at the sending end 

 of the cable it is most convenient to compute the attenuation of the 

 cable for currents so small that Rh may be safely neglected. The 

 attenuation thus computed is that which would be obtained over 

 the whole cal)Ie if a \(t\ small sending voltage were used. The 

 additional attenuation at tiie sending end for the desired sending 

 voltage ma\- then be appro.ximated by computing successively from 

 the sending end the attenuation of short lengths of cable over which 

 the current amplitude may be considered ci>nstant, the attenuations 

 of separate lengths being added together to gi\e the attenuation of 

 that part of the cable in which hysteresis cannot be neglected. In 

 this computation account must, of course, be taken of the increased 

 inductance and eddy current resistance accom|xui\ing the higher 

 currents at the sending end. 



Having calculated or obtained by measurement the sc\eral resist- 

 ance factors and knowing the capacity, leakance and inductance, the 

 whole attenuation of a cable for any desired frequency may be com- 

 puted and a curve drawn showing the variation of received current 

 with frequency for a given sending voltage. This relation for a 

 particular case is shown in Curve c of Fig. 4. Cur\e a shows for 

 comparison the relation between frequency and received current of a 

 non-loaded cable of the same size, that is, a cable ha\ing a conductor 

 diameter the same as that of the loaded conductor and having the 

 same weight of gutta percha. Curve b shows the behavior of an 

 ideal loaded cable having tlie same inductance, ca|)acity and d.c. 

 resistance as the real loaded cable of Curve c, but in which the leakance 

 and alternating current increments of resistance are assumed to be zero. 



Now, if the level of interference through which the current must 

 be received is known, the maximum speed of signalling for the loaded 

 cable may be obtained from Curve c It is that speed at which the 



