HIGH-SPEED OCEAN CABLE TELEGRAPHY 237 



Standpoint have been the size of the conductor and the thickness of 

 insulating material. The electrical design of a loaded cable is, how- 

 ever, somewhat more complicated since in addition to copper resistance 

 and electrostatic capacity we have here to be concerned with the 

 inductance added by the loading material and also with added re- 

 sistance factors which are introduced by its use. The problem of 

 electrical design, therefore, involves determining not only the size of 

 the copper conductor, but also the electrical and magnetic charac- 

 teristics and the shape and dimensions of the loading material, as 

 well as the electrical characteristics of the insulatmg material, which 

 will give the highest speed of operation consistent with the mechanical 

 and cost limitations which are imposed. 



Since the object of the electrical design is to secure high operating 

 speed, it is essential to consider what are the factors which limit speed 

 and how they are taken into account. This subject has already been 

 treated in some detail in previous papers,^ and only a general review 

 of the principal factors involved in the electrical design will be under- 

 taken in the present paper. 



In the history of cable development prior to the introduction of 

 the permalloy-loaded cable various physical factors at different times 

 limited the speed of operation which could be obtained with a long 

 ocean cable. These were principally distortion of signals, sensitivity 

 of receiving apparatus, limited safe sending voltage, inaccuracy of 

 duplex balance, and extraneous interference from both natural and 

 man-made sources. With the development of cable amplifiers and of 

 improved means of signal shaping, the factors of distortion and limited 

 sensitivity of receiving apparatus were effectually eliminated and at 

 the time when the development of the permalloy-loaded cable was 

 undertaken the speed of long cables was limited in most cases by the 

 accuracy with which artificial lines could be made to balance cables in 

 duplex operation. In some cases where extraneous interference was 

 unusually severe the limit of speed was set by that factor combined 

 with the limit of sending voltage which was usually placed at about 

 50 volts by extreme concern for the safety of the cable insulation. 



It was by no means obvious which of these several factors should 

 be considered in the electrical design of a loaded cable. With the 

 vacuum-tube amplifier available to amplify the weak received signal 

 to the degree necessary to operate recording mechanisms, there was 

 no practical limit to the sensitiveness of receiving apparatus. It was, 

 however, necessary to consider distortion as a possible limit to speed. 



^O. E. Buckley, Jour. A. I. E. E., Vol. XLIV, pp. 821-829, August 1925, 

 B. S. T. J., Vol. IV, No. 3, pp. 355-374, July 1925; J. J. Gilbert, B. S. T. J., July 

 1927. 



