368 BELL SYSTEM TECHNICAL JOURNAL 



loading material if a benefit is to be obtained from continuous loading. 

 The addition of the loading material has two oppositely directed 

 effects; on the one hand it tends to improve transmission by increasing 

 the inductance and consequently decreasing the attenuation, and 

 on the other hand it tends to increase the attenuation by increasing 

 the effect of leakance and by the addition of resistance. Not only 

 are the hysteresis and eddy-current factors of resistance added by 

 the loading material but it must also be looked upon as increasing 

 either the copper resistance or the capacity on account of the space it 

 occupies. Generally it is more convenient to look upon the loading 

 material as replacing some of the copper conductor in the non-loaded 

 cable with which comparison is made, since by so doing all of the 

 factors outside of the loaded conductor are unchanged. Now, if the 

 loading material is to be of any benefit, the decrease in attenuation 

 due to added inductance must more than offset the increase due in 

 added resistance, including the added copper resistance due to the 

 substitution of loading material for copper. In the limiting case the 

 lowest jjermeability material which will show a theoretical advantage 

 from this point of view is that which, as applied in a vanishingly thin 

 layer, gives more gain than loss. For any particular size and length 

 of cable there is a limiting value of permeability which will satisfy 

 this condition, this limiting value being greater the longer the cable 

 and the smaller the diameter of its conductor.' For transatlantic 

 cables of sizes laid prior to 1923 the minimum initial permeability 

 required to show an advantage is higher than that of any material 

 known prior to the invention of permalloy. Actually a considerably 

 higher permeability than this theoretical minimum was, of course, 

 required to make loading an economic advantage since there are 

 practical limits to the thickness of loading material and since the cost 

 of applying it has also to be taken into account. Further, there are 

 limits on methods of operation imposed by loading which necessi- 

 tate still higher permeability to make loading worth while. 



Since the addition of loading has two opposite tendencies in its 

 effect on attenuation, the practical design of the cable must be based 

 on a compromise between them. Thus, to secure the maxinuini 

 gain from loading a cable of a given size, the loading material should 

 be chosen of such a thickness that the gain due to increased induct- 

 ance from a slight increase of thickness just offsets the loss due to 

 increased resistance and dielectric leakance. In practice, of course, 

 economic considerations of the cost of \arious thicknesses of loading 

 must also be taken into account. 



•See British Patent No. 184,774—1923, to O. E. Buckley. 



