HIGH-SPEED OCEAN CABLE TELEGRAPHY 239 



experience has well justified this procedure for the cables which have 

 been made. 



The problem of designing a loaded cable was thus reduced to 

 proportioning its component parts so as to secure the desired speed of 

 operation under the conditions imposed by the limitations of sending 

 voltage and received interference. Considerations of safety limit the 

 sending voltage to about 50 volts, and terminal interference as ordi- 

 narily experienced requires that the received signal shall have an 

 amplitude of a few millivolts. The risk of increasing the sending 

 voltage to several hundred volts would not necessarily be serious but 

 little advantage could be gained by taking this risk since, with the 

 materials and type of construction used, higher sending voltage would 

 involve increased hysteresis and eddy-current losses and consequently 

 would not result in a proportionately higher received voltage. It is, 

 however, possible to reduce the received interference by proper 

 termination and this is of great importance in cases where the inter- 

 ference is severe. 



The nature of cable interference and methods of reducing it have 

 been discussed in a paper by J. J. Gilbert^ in which is described the 

 method which has been used to decrease the terminal interference on 

 the loaded cables which have been laid. This method consists in 

 using, as the earth connection for the receiving apparatus, a "balanced " 

 sea-earth, terminating in deep water. With ordinary cables the 

 common practice has been to provide as the earth connection a sea- 

 earth core, similar to the main core and sheathed with it, but extending 

 only a few miles from shore to a point where the sea-earth conductor 

 is connected to the sheath of the cable. While this type of earth 

 greatly reduces the interference picked up in and near the cable 

 terminal, it does not completely eliminate it. Almost complete 

 elimination of the effects of disturbances originating between the 

 termination of the sea-earth core and the shore may be obtained by 

 providing a terminal impedance between the sea end of the sea-earth 

 conductor and the sheath of the cable. For a non-loaded cable a 

 combination of condensers and resistances would be required to 

 make up such a terminal impedance, but for the loaded cable a very 

 close approximation is secured by a simple resistance of a few hundred 

 ohms. A few hundred feet of manganin wire, insulated like the rest 

 of the conductor and joined to the end of the sea-earth core, serves 

 this purpose admirably. This type of construction has been used on 

 the New York end of the New York-Horta and on all terminals of the 



* J- J. Gilbert, B. S. T. J., Vol. V, No. 3, pp. 404^17, July 1926. See also 

 Electrician, Vol. 97, p. 152, August 1926. 



