406 BELL SYSTEM TECHNICAL JOURNAL 



arising ijcom. a variety of sources, which may be broadly grouped into 

 two classes. The first class comprises artificial sources, such as elec- 

 trical power or railway systems in the neighborhood of the cable ter- 

 minals. Currents circulating between the various earth connections of 

 such systems give rise to electromagnetic fields in the earth and sea 

 water, which fields may have the form of transient surges or pulses, 

 or may be periodic in nature. The second class includes the various 

 manifestations in the atmosphere or at the surface of the earth, such 

 as electric or magnetic storms, which are also responsible for the dis- 

 turbances in radio communication known as "static." \'ery little 

 definite data is available regarding the magnitude and character of 

 the natural disturbances affecting submarine cables, but it is found 

 that, as in the case of static, the intensity of such effects is influenced 

 by a number of factors, the season of the year and the geographical 

 location being among the most important. At times of unusual activ- 

 ity, such as that accompanying the aurora polaris or local electrical 

 storms, the voltages induced in the cable conductor are so large as to 

 prohibit operation of the cable. 



Except in the case where the source is in the immediate vicinity of 

 the cable, the effect of any disturbance upon the cable can be con- 

 sidered as the result of a fluctuation of potential at the surface of a 

 massive conducting medium, the ocean, which gives rise to electromag- 

 netic waves which are propagated in all directions from the source and 

 which penetrate the interior of the conducting medium according to the 

 well-known laws governing "skin effect." Due to the presence of 

 varying electric and magnetic fields in the sea water adjacent to the 

 cable, an electromotive force is induced in each section of the cable 

 conductor, and the resulting current is transmitted along the con- 

 ductor to the cable terminal, combining with the currents due to elec- 

 tromotive forces induced in other sections to make up the total ex- 

 traneous interference. 



At the surface of the ocean the disturbance may take a variety of 

 forms, for instance a succession of pulses or a train of damped oscilla- 

 tions. In any case the most convenient method of following the dis- 

 turbance through the sea water into the cable conductor and along 

 the conductor to the cable terminal is to consider the disturbance 

 made up of a number of sinusoidal components of all frequencies from 

 zero to infinity, the relative amplitudes and phases of the various 

 components being determinable from the wave shape of the disturbance 

 by the methods of Fourier analysis. The transmission characteristics 

 of the interference transmission system at any particular frequency can 

 then easily be studied, and finally the total effect of the original dis- 



