THE LOCALISATION OF BREAKS AND FAULTS. 505 



Elimination of Earth Current Effect by False Zero. — In 

 the false zero method the earth current deflection should 

 be observed after as well as before the bridge balance in order 

 to follow any changes in the earth current. It should also be 

 remembered that the effect of the earth current on the galvano- 

 meter varies with the resistance unplugged In the third arm of 

 the bridge. Consequently, the first observation of false zero 

 will not be so correct as that made when the resistance un- 

 plugged is approximately a balance. And the closer the 

 bridge is balanced the more correct will the false zero be. 

 Hence towards the end of the test, when the balances are 

 more alike and steady, the false zero must be more carefully 

 worked to. The observation of false zero is made with the 

 galvanometer short-circuit key opened and the testing key at 

 rest, so connecting the bridge fork to earth. A short interval 

 must be allowed to elapse after taking off the testing current 

 and before the false zero is observed in order to let the capacity 

 discharge pass. The same interval must be allowed for the 

 charge to pass after putting on the testing current and before 

 opening the galvanometer. If the earth current is very strong 

 the true zero of the spot may be adjusted off the scale, so that 

 the false zero is somewhere on the scale for conveuient obser- 

 vation. A high resistance may be added in circuit with the 

 galvanometer if too sensitive. This is better than using a 

 shunt in false zero work. 



Elimination of Earth Current Efect hy Reversals. — If the 

 earth current is very strong false zero observations necessitate 

 the galvanometer being considerably reduced in sensitiveness. 

 Reversals to true zero may then be used with advantage as the 

 instrument is in a more sensitive state and better appreciation 

 is obtained. But it must be remembered that observations by 

 reversals are only correct when the earth current does not alter 

 during a pair of balances. Each pair of observations should 

 therefore be taken quickly in succession to minimise as far as 

 possible any change in the earth current. When several pairs 

 come out alike they may be accepted as having fulfilled this 

 condition. In this teat no change is made in the battery power, 

 the same number of cells being used throughout. 



An earth current in a cable under test, or rather the differ- 

 ence of potential which gives rise to it, acts either against or 



