THE LOCALISATION OF BKEAKS AND FAULTS. 401 



apparent resistances due to polarisation and earth current are 

 shown as a separate part of this line because they are sepa- 

 rately allowed for and eliminated by reading to false zero. The 

 lower line represents the resistances as tested with the higher 

 current nc, in which the line resistance is unaltered but the 

 exposure resistance is reduced according to Kennelly's law. 

 The polarisation and earth-current resistances are also shown 

 separately and reduced in proportion to the larger current. 



This test is capable of very reliable localisations on longdis- 

 tance breaks. The fall in the polarisation potential is some- 

 what delayed by the static discharge with a fairly long length 

 in circuit, and an interval of a second or two may be allowed 

 before opening the galvanometer to observe false zero which 

 greatly helps in obtaining correct readings of the zero and 

 balances thereto. The correct interval of time to allow depends 

 upon the length of cable in circuit. In this, as in all 



v^ , , nc 



Fig. 236. — Analysis of Kennelly's Two-current Test to False Zero. 



localisation work, experience and observation are the best 

 teachers, and with plenty of practice something seems to guide 

 one instinctively as to the right time to allow when testing 

 breaks on long sections. It is also due to the introduction of the 

 Sullivan dead-beat galvanometer that false zero work has been 

 immensely facilitated and rendered a fairly easy operation. 



On short lengths the fall of the polarisation potential is very 

 rapid and there is rather more uncertainty in rightly timing 

 and observing the immediate false zero, owing to the greater 

 interference of the discharge. 



Kennelly's test may be taken by direct deflection, comparing 

 the cable deflection with that through a known resistance, or by 

 adjusting the known resistance to reproduce the deflection. A 

 dead-beat galvanometer is required, for which the Sullivan 

 instrument is very suitable, shunted with a low resistance of 

 one ohm or thereabouts. Only one instrument is required, as 

 against two when taking this test on the bridge. A two-way 



