CHAPTER V. 



THE LOCALISATION OF BREAKS AND FAULTS. 



In shallow water cables the covering of brass tape by which 

 the core is protected from the teredo prevents the rapid de- 

 velopment of a fault under the action of the current, because 

 the gas evolved has no free escape, and becomes occluded on 

 the surface of the tape. The result is that faults in these 

 cables are of high resistance and often difficult to locate. 



In duplicate cables a fault, or even a total interruption of 

 one cable, does not trouble the public, whose messages still go 

 through by the other; but it is a matter requiring prompt 

 action on the part of the owners, in order to keep the dupli- 

 cation intact ; and in single cables worked duplex a very small 

 and variable fault will put the balance out of the range of 

 adjustment. Consequently, faults are dealt with as soon 

 as they occur, and are oftener than ever of a very minute 

 character. Such faults require very rapid observations on the 

 instruments on account of polarisation, and this action is so in- 

 stantaneous in some cases as to render the results from one end 

 quite unreliable ; but, notwithstanding these difficulties, the 

 smartness with which repairs are carried out to-day is proof 

 that considerable practical advances have been made. 



Faults are sometimes so small as not to have the slightest 

 effect on the signals through a cable worked simplex, and, 

 indeed, would not be noticed at all if it were not for their 

 effect on the duplex balance and the periodical tests. Other 



