of determining Resistances. 



443 



be put to earth, when required, by an attendant stationed at 

 that end. 



The coil or shunt of known value should be, preferably, equal 

 to that of the conductor of the cable. The battery-power re- 

 quired must depend on the resistances of the fault and conductor 

 of the cable. The resistance of the conductor must be carefully 

 ascertained. 



In the case of iron-covered cables, it will be difficult to connect 

 the ends of the cable to the galvanometer terminals without in- 

 terfering with the zero-position of the needles. The needles 

 may be deflected by a current set up between the copper con- 

 ductor and the iron sheathing of the cable, or even by the tanks 

 themselves. 



When the resistance of the fault is low, the deflected position 

 of the needle may be taken as zero, or the needle may be set to 

 zero by the aid of other magnets. Care must be taken that the 

 electromotive force of this current shall be insignificant when 

 compared with that of the testing-battery. 



Supposing the resistances of the galvanometer and conductor 

 to be known, and the two ends of the cable to be led into the 

 testing-room, the connexions will be made alternately as shown 

 in figs. 1 and 2. 



Galvanometer. Galvanometer. 



Let R = resistance of conductor A B, 

 G = „ galvanometer, 



r = „ from end A to x, including x, 



r J = „ „ B to x, including x, 



x being the resistance of the fault. 



If a condenser, first charged from a constant battery, be dis- 

 charged through the galvanometer shunted with R, r, r' conse- 

 cutively, the deflections will be proportional to the shunt- powers 

 of these resistances respectively, or to 



G + R G + r G+r' 



R 



r 

 2G2 



