548 SUBMARINE CABLE LAYING AND REPAIEING. 



equalisation of instruments or deflections is required, and the 

 method involves no more than an ordinary deflection test. 



In practice this method has been very successful in localising 

 difficult high resistance variable faults, especially phantom 

 faults, which appear and disappear when least expected. In one 

 instance, in a cable 2,300 miles long, a fault of this description 

 disappeared so as to permit of localising a break 300 miles 

 beyond it by a capacity test taken across it. This fault, which 

 varied between 7,000 ohms and 20 megohms, was localised 

 within 9 miles by a single series of simultaneous observations 

 on Betts' method, while another similar fault was localised 

 within 4 miles. These were exceptionally difficult phantom 

 faults on long cables, previous localisations by all other known 

 methods and by experienced hands yielding widely different 

 and totally unreliable results. 



The Author has suggested using milammeters in the line, so 

 dispensing with the necessity of taking galvanometer constants- 

 or converting deflections into equivalent resistances. Several 

 pairs of readings would be taken on the milammeters by A and 

 B at exactly the same instant of time by arrangement. 



Let c =milliamperes at station B. 

 Let nc= „ „ A. 



Then 



■nc Jj — x 



c X 



and the distance of the fault is 



x=^, . (6) 



To include the correction for battery and milammeter resis- 

 tance at each end if these quantities cannot be neglected 

 formula (6) becomes 



^J. + r,-rn ^^^ 



n+1 



The ratio of the currents (n) will be greater than unity 

 when the fault is nearer station A and less than unity when 

 It is nearer station B. 



Where a duplicate cable exists this test is a valuable check 

 to the loop, the second cable being used for communicating 



