Extraneous Interference on Submarine 

 Telegraph Cables 



By J. J. GILBERT 



Synopsis: In order to avoid a considerable reduction iii speed of opera- 

 tion, which would have resulted on account of the unusually large parasitic 

 disturbances encountered in the neighborhood of New York, the New 

 York-Azores permalloy loaded cable was equipped with a new type of earth 

 connection consisting of a conductor extending 10() nautical miles to sea 

 and there connected to earth through an artificial line. 



This paper presents the theory of the new type of sea earthing arrange- 

 ment and discusses the sources of extraneous interference and the manner 

 in which it is picked up by submarine cables. A method is developed for 

 estimating the magnitude of terminal extraneous interference in the case of 

 any particular cable. 



AMONG the factors limiting the speed of operation of long sub- 

 . marine telegraph cables one of the most important is the mutila- 

 tion of the received signals by electrical disturbances picked up along 

 the cable and transmitted with the incoming signal to the receiving 

 instrument. The nature of this disturbance is shown in Fig. 1 which is 

 an oscillographic record over a short period of time of the difference of 

 potential across the terminals of the receiving instrument of a cable 

 system, at a time when no signals were being received over the cable. 

 Although the complete signal correction networks were not in circuit 

 at the time this record was taken, the latter is representative of the 

 form of the extraneous disturbance that would be superposed on the 

 record of an incoming signal. It is evident that unless the signal am- 

 plitude is sufficiently large compared with the amplitude of interfer- 

 ence, the latter will seriously interfere with the interpretation of the 

 siphon recorder tape or with the functioning of relays operated by the 

 signal current. That this condition constitutes a limit on the speed of 

 operation of the cable is indicated by Fig. 2 which shows the amplitude 

 of a signal, received over a typical transatlantic cable, as a function of 



Z MILLIVOLTS 



O. / SECOND 



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