THE LOCALISATION OF BREAKS AND FAULTS. 393 



ohms in resistance. When carbon brings the resistance down 

 It is probably due to the setting free of occluded gas at the 

 exposure by the formation of chloride. It sometimes happens 

 that tests taken immediately af ber a break has occurred differ 

 by some miles from those that are obtained an hour or so later. 

 This frequently means that the end has had time to become 

 buried in mud. 



A break does not always leave a large exposed end. Some- 

 times it is of very high resistance, due to the percha being 

 drawn over it, but in any case a break is evident by interrupted 

 communication. As it may sometimes behave as a high resis- 

 tance or partial earth fault, and faults behave as breaks in 

 respect to the area of copper exposed, the various localisation 

 tests have come to be regarded from the point of view as to 

 their applicability to large or small exposures irrespective of 

 whether there is an actual fracture or not. It should there- 

 fore be borne in mind that methods known as break tests are 

 frequently applicable to partial interruptions and vice versa, 

 and experience can alone decide from the first observations 

 and the behaviour of the fault upon which methods most 

 reliance can be placed. A Mance test, for instance, although 

 devised as a partial earth test, gives a very good localisation 

 on a break fairly close in when the resistance of the break 

 is allowed for, and a Schaefer, although primarily a break test, 

 gives excellent results on a fault of fair exposure. Results of 

 equations must not be accepted on rough tests alone, but if 

 time is short and it is required to get an approximation of the 

 length in circuit without exact localisation, a couple of plain 

 bridge readings to false zero, zinc followed by carbon, gives 

 vastly more information than a dozen results of formulee. 

 Eads are very diverse in their behaviour and it is not always 

 advisable to use carbon except as a final act, as it is liable to 

 spoil the end. The Mance test is a trustworthy method that 

 will reveal the lowest possible bridge reading, and that is a 

 great deal of information to go upon. 



Two-current Kennellys depend to a greater extent probably 

 than any other tests upon the manipulator, but in good hands 

 very close localisations of breaks are obtained, especially when 

 a long distance off. If out at all, it generally brings the break 

 home a little but when results by this and Schaefer's methods 



