THE LOCALISATION OF BREAKS AND FAULTS. 405 



meter principle, providing various ranges for reading direct up 

 to 300 milliamperes in divisions on the scale (see page 534) 

 Ttie milammeter may be used instead of the shunted Sullivan^ 

 in which case the currents are read direct. 



Kennelly's Three-Current Break Test to True Zero. — 



Dr. Kennelljj in his Paper " On the Localisation of a Complete 

 Fracture in a Submarine Cable by Bridge Measurement to 

 Instrument Zero " (The Electrician, Oct. 14 and 21, 1887), 

 showed that by three consecutive bridge balances to instrument 

 or scale zero, the break exposure and the equivalent earth 

 current resistances were eliminated. The connections are the 

 same as in Fig. 235. 



Let A be the balance with the lowest current c, 

 „ B „ ,, ,, intermediate current 4c, 



„ C „ „ ,, highest current 9c. 



Using testing currents in the proportion of 9:4;1, the^ 

 formula for distance of break is : — 



a. = ^-4(B-C). 



When the ratio of currents used is 16; 4:1 (the reading C 

 being with current = 16c), the distance of break is : — 



a; = ^+^^-2(B-C). 



When the ratio of currents is 4:2-25:1 (the reading C being 

 with current = 4c), the distance of break is : — 



r» = 2A-9B + 8C. 



In these formulee Dr. Kennelly has taken into account the 

 variation in resistance of the fault with the different strengths 

 of currents used. That is, on the basis of the law which he 

 established by experiment — that the resistance of an exposed 

 conductor varies as the square root of the current. This law, 

 however, was not found to hold good in very small exposures, 

 or where any obstruction such as copper, salts, sand or mud, 

 prevented free contact between the conductor and the sea. 

 Therefore, some error may come in if the exposure is very 

 small at the fracture, or the end buried or partially obstructed. 



dd2 



