258 BELL SYSTEM TECHNICAL JOURNAL 



Electrical Measurements of Loaded Cables 



To check the assumptions made in the design of the first cables, 

 and to obtain the information necessary for the design of the ultimate 

 operating equipment, extensive electrical measurements were made 

 on the three Western Union cables after they had been laid. From 

 an analysis of these measurements the several electrical parameters 

 of the cables were determined. To do this required new apparatus 

 and methods, the development of which was by no means a small 

 part of the total effort involved in the first project. Since a review 

 of some of the methods of measurement has been given in a recent 

 paper by J. J. Gilbert,^" the present discussion will be limited to the 

 apparatus and methods which seem to be of particular interest. 



One of the most important tools in all our investigations concerned 

 with the permalloy-loaded cable was the string oscillograph shown 

 in Fig. 13. From the start it was recognized that an instrument 

 would be needed which would give an accurate record of the manner 

 in which the currents and voltages which were being studied changed 

 with time and, in fact, the first step in the experimental investigation 

 of the cable problem was to search for a suitable oscillograph. For- 

 tunately it was not necessary to look far. A string oscillograph which 

 had been developed for sound-ranging of artillery during the World 

 War was quickly modified and devoted to more peaceful purposes. 

 The present instrument differs in many details from the original but 

 retains the invaluable asset of ability to give almost instantaneously, 

 completely developed and fixed, a distortionless picture of a wave 

 involving any frequencies up to about 300 cycles per second. In a 

 study like that of signal shaping, involving the determination of the 

 efi"ect of numerous slight changes in adjustment of the apparatus, the 

 advantage of such an instrument is obvious. This instrument was 

 used both in the early studies of signal correction with the laboratory 

 artificial cable and in the later measurements on the laid cables, and 

 today is a useful adjunct in cable stations where it serves to show the 

 character of the cable signals at any stage of their conversion into 

 impulses for the recording instruments. A feature of particular value 

 in studying phenomena such as extraneous interference on cables is 

 that the oscillograph permits taking a continuous record over a 

 period of several minutes. 



Other special instruments which I shall only mention, but which 

 were developed especially for the cable experiments, were an at- 

 tenuation meter and a low-frequency vacuum-tube oscillator to give 



*"" Determination of Electrical Characteristics of Loaded Telegraph Cables," 

 B. S. T. J., July J 927. 



