HIGH-SPEED OCEAN CABLE TELEGRAPHY 241 



The desirable qualities in the loading material from the electrical 

 point of view are high initial permeability, high resistivity and 

 constancy of permeability in the range of magnetizing forces con- 

 cerned. The exact composition of permalloy which would give the 

 best combination of these properties would, of course, be different 

 for different cables but for practical reasons it is desirable to choose 

 a composition which approximates the optimum for general use. 

 Having determined on a particular alloy, the optimum size of con- 

 ductor and thickness of loading material may readily be computed 

 on the basis of its known electrical and magnetic characteristics. 

 With the compositions of permalloy which have been used, the 

 optimum thickness of the layer of permalloy for a long ocean cable 

 generally lies in the range from 0.005 inch to 0.010 inch which is 

 fortunately convenient from the mechanical point of view. If less 

 than the optimum thickness is assumed, the inductance will be too 

 low and the consequent required conductor diameter will be too large. 

 On the other hand, if more than the optimum thickness is assumed, the 

 increase of eddy-current resistance and the effect of dielectric leakance 

 will more than offset the gain due to the increased inductance. 



In determining the optimum thickness of the permalloy it is, of 

 course, essential to include all the resistance factors which are of 

 consequence. In addition to eddy-current resistance and the effect of 

 dielectric leakance there are the factors of hysteresis resistance and 

 sea-return resistance which must, in particular, be taken into account. 



The effect of hysteresis on attenuation is felt only near the sending 

 end of the cable since over most of the length of the cable the current 

 is so small that the hysteresis is negligible. Its effect near the terminals 

 may be calculated by the method of successive approximations which 

 takes account of the falling off of current and the change of hysteresis 

 resistance with current amplitude. Ordinarily the effect of hysteresis 

 becomes negligible beyond the first one or two hundred miles from 

 the sending terminal. Within that range it may add as much as 

 10 TU to the total attenuation of the cable for the high-frequency 

 components of the signals. 



By sea-return resistance is meant the resistance which is contributed 

 by the sea water and armor wire around the core of the cable. In low- 

 speed non-loaded cables this factor may be safely neglected since the 

 return current of low-frequency signals spreads out through such a 

 great area around the cable that the resistance contributed by the sea 

 water is negligible. With the high-frequency signals of the loaded 

 cable, however, the return current tends to concentrate in the sea 

 water close to the cable and much of it flows in the armor wires. 

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