THE LOCALISATION OF BKEAKS AND FAULTS. 395 



it. The subject was first made public in a Paper by Dr. 

 Kennelly before the then Society of Telegraph Engineers and 

 Electricians in March, 1887, on "The Resistance of Faults in 

 Submarine Cables." The practical conditions and limitations 

 may be stated as follows : — 



The law is based upon gas development at the break, and 

 has apparently its origin in this gas and its rate of discharge 

 from a perfectly clean exposure. The resistance at the break 

 involved by this law is therefore a function of the testing 

 current. If the area of bare copper conductor exposed at the 

 break is very small or partially covered by the broken ends of 

 the percha, or coated with copper salts, or buried in sand or 

 mud, the gas will not escape freely; but accumulate on the 

 obstructions, and when this is so the law does not hold good. 

 Such mechanical obstructions or insulations are in the nature 

 of haphazard resistances, which do not follow any law and 

 which it is impossible to eliminate by any test or formula from 

 the conductor resistance proper. When obstructed in this 

 way the distance of the break by this test comes out too high 

 because the exposure resistance then varies as a higher power 

 of the current. 



On account of the difficulty in measuring the true earth and 

 polarisation currents active in the cable during the test, an 

 error is introduced which makes the result somewhat too high. 

 The deflection caused by these currents is read oq the galvano- 

 meter as soon after breaking the testing current as possible, 

 and the position on the scale treated as the false zero ; but 

 some seconds elapse before it can be observed, in consequence 

 of the electrostatic discharge from the cable. And, as the 

 polarisation potential rapidly falls, the correct false zero is not 

 definitely determinable. The error, however, is almost inap- 

 preciable if the strength of current used in the test does not 

 exceed 25 milliamperes. 



On this point Dr. Kennelly says : " A certain zero is 

 assumed to start with as the false zero to work to ; namely, 

 the polarisation or cable current then existing. The battery 

 key is then depressed and balance adjusted till this zero ia 

 maintained. The key is then released. Lat X represent 

 time and Y the observed discharge on the galvanometer 

 (Fig. 234). Now, assuming the truth of the law, it can 



