390 BELL SYSTEM TECHNICAL JOURNAL 



accurately determined. If, for example, the interference in the case 

 of the cable having the attenuation-frequency characteristic shown in 

 curve B were found to be twice as great as had been anticipated, the 

 amplitude of received signal would likewise have to be doubled, which 

 would mean a reduction of 0.7 in the allowable attenuation constant. 

 This, as can be seen from curve B, would correspond to a reduction 

 in speed of 8 to lO c.p.s. Also since the value of attenuation constant 

 is considerably affected by variations of the electrical parameters, 

 it is desirable that the values of these parameters in the laid cable be 

 capable of predetermination to a degree of accuracy comparable with 

 that obtained in the case of non-loaded cables. Methods of estimating 

 the value of extraneous interference to be expected at the terminals 

 of a projected cable have been described in a previous paper.'^ The 

 present paper will be devoted to a discussion of methods of pre- 

 determining the electrical parameters of cables. 



Measurements during Manufacture 



In the case of a non-loaded cable the attenuation constant, as 

 indicated by formula (3), is determined solely by the dielectric capacity 

 and the conductor resistance. For the values of frequency involved 

 in the operation of such cables, the latter consists almost entirely of 

 the direct current resistance of the copper conductor. The values of 

 capacity and copper resistance of a considerable part of the cable can 

 be measured during the process of manufacture, and, by reducing 

 these values to sea bottom conditions, an accurate estimate of the 

 resistance and capacity of the laid cable is obtained. 



In the case of the loaded cable the problem of predetermining the 

 electrical parameters of the laid cable is much more difficult, since a 

 number of the quantities involved in computing the attenuation are 

 influenced by conditions which are not entirely known and which 

 are difficult to simulate in laboratory experiments. The dielectric 

 leakance, for example, is affected by pressure as well as by temperature, 

 and since the hydrostatic pressure to which the cable is subjected may 

 be as high as io,ooo pounds per square inch, it is evident that measure- 

 ments of this characteristic of the cable, on any but a very small 

 scale basis, will be very difficult and costly. The permeability of the 

 loading material and consequently the inductance of the cable may 

 be affected by mechanical strain and by superposed magnetic fields. 

 An estimate of the average inductance of the laid cable can be obtained 

 by bridge measurements in the factory on pieces of core about 1 



3 J. J. Gilbert, B. S. T. J., Vol. 5, p. 404, and Electrician, Vol. 97, August 6, 1926. 



