Determination of Electrical Characteristics of Loaded 

 Telegraph Cables 



By J. J. GILBERT 



Synopsis: The use of permalloy for continuous loading has introduced 

 a number of new factors of importance in the study of transmission of 

 signals over long submarine telegraph cables. Data to check the theo- 

 retical assumptions that are used in the design of permalloy loaded cables 

 can be obtained by measuring on such cables the attenuation and time 

 of propagation of sinusoidal currents of various frequencies in the telegraph 

 range. By combining the results of these measurements with data obtained 

 on the cable during process of manufacture, the resistance, inductance, 

 capacity and leakance of the cables can be determined. 



This paper describes the experiments that were performed on three 

 laid cables and discusses in a general way the methods of computing the 

 cable parameters. 



WITHIN the last few years the art of telegraphing over submarine 

 cables of transoceanic length has been revolutionized by the 

 development of effective means of applying to such cables the principle 

 of inductive loading. By surrounding the copper conductor of the 

 cable with a thin layer of permalloy, a material of high magnetic 

 permeability, the range of signal speeds attainable over cables of the 

 order of 2,000 n.m. in length has been multiplied eight to ten times. ^ 

 In place of the low frequency band extending from zero to about 

 15 c.p.s., which represents the range of frequencies which can be 

 efficiently transmitted over the usual type of non-loaded cable, we are 

 concerned in the case of the loaded cable with a transmission band 

 extending from zero to about 120 c.p.s. Largely because of this 

 comparatively high speed of operation, a number of factors, which 

 were of negligible influence in the case of non-loaded cables, have 

 become of primary importance in affecting the speed of signalling, 

 and it has been found necessary, in order to establish a definite basis 

 of estimating the performance of loaded cables, to make a thorough 

 study of these factors by theoretical analysis supplemented by experi- 

 mental work in the laboratory, and by measurements on laid cables. 



Principles of Cable Transmission 



The theory of transmission of signals over submarine telegraph 

 cables - and the principles governing the design of permalloy loaded 



> O. E. Buckley, B. S. T. J., Vol. IV, No. 3, July 1925; Electrical Communication, 

 Vol. 4, No. 1, July 1925; Jour. A. I. E.E., Vol. XLIV, No. 8, August 1925. 



'^ H. W. Malcolm, "The Theory of the Submarine Telegraph and Telephone 

 Cable," London, 1917. 



J. W. Milnor, Jour. A. I. E. E., Vol. 41, p. 118, 1922. 



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