INTERFERENCE ON SUBMARINE TELEGRAPH CABLES 413 



reacted in the same degree to these disturbances, then it is clear that 

 corresponding to each disturbing impulse at T\ due to pick-up at any 

 point ai on TiM there would be an equal impulse at T2 due to pick-up 

 at a2 the corresponding point on T2 G and there would be no resulting 

 difference of potential impressed on the receiving network due to these 

 disturbances. As a matter of fact the section T2 G does not react to 

 disturbances in the same manner as the section Ti M, even though the 

 two cores have identical linear characteristics. Although the imped- 

 ances looking landward from ai and ^2 will be equal, the impedances 

 looking seaward from the two points are likely to be widely different, 

 and the impedances into which electromotive forces induced at ai and 

 a2 work will not be equal. The same disturbance will therefore set up 

 currents of different amplitudes in the two conductors, and there will 

 be a difference of potential between 7\ and To which will be indicated 

 on the receiving instrument. Another way of looking at this effect is 

 to consider the disturbances picked up at ai and 02 as resulting in 

 transient waves of potential and current which are propagated along 

 the two conductors in both directions from the points of pick-up. The 

 waves travelling from ai to Ti are equal to the corresponding waves 

 travelling from 02 to Ti- A similar equality holds for the waves travel- 

 ling from a2 to G and from Oi to M. On arriving at G the waves on the 

 sea earth conductor are reflected and travel back along the conductor, 

 finally arriving at T2. Since there is no corresponding reflection on 

 the main conductor, there will be an unbalanced disturbance, the 

 magnitude of which depends upon the amount by which the disturb- 

 ance was attenuated in travelling over the route ai — G—T^. 



The remedy '^ for the condition just described is to eliminate reflec- 

 tion at the sea end of the sea earth conductor, or, if for any reason, 

 there is a reflection at the point M, to balance it with an equal reflection 

 at the point G. This can be done by grounding the sea earth con- 

 ductor at G through a network having an impedance that bears the 

 same relation to the impedance of the conductor G T2 as the impedance 

 of the cable seaward of M bears to the impedance of the conductor 

 M Ti. When the two cores Ti AI and T2 G are alike, the impedance 

 of the network should equal the impedance of the main cable at Af.^ 



s Osborne, U. S. Patent 1,390,580—1921. 

 Heurtley, Br. Patent 198,978—1923. 

 Gilbert, Br. Patent, 218,261—1926. 



^ There is one important type of disturbance which has not been dealt with in the 

 preceding discussion, namely, that due to the signal currents on cables which cross or 

 lie close to the cable in which we are interested. It is evident that the electromotive 

 forces induced in the cal)le conductor due to such causes behave in the same manner 

 as any other disturbing electromotive force and that the magnitude of their effect 

 can be reduced by the use of a balanced sea earth conductor terminated at a point 

 beyond the region of disturbance. 



