THE LOCALISATION OF BREAKS AND FAULTS. 



533 



The best method to adopt is Loi-d Kelvin's bridge test 

 {Fig. 301) usually known as Thomson's method. The gdlvano- 

 meter is placed in the unknown resistance arm and its usual 

 place across the bridge taken by a plain connecting wire with 

 key. The battery is in the usual place, and if the galvano- 

 meter is a reflecting one, as will usually be the case, the cur- 

 rent supplied must be very minute. Xot more than one cell 

 must be used, and this should be baffled down in the manner 

 recommended by Messrs. Fisher and Darby in their thoroughly 

 practical work •' Students' Guide to Submarine Cable Testing." 



'-WVNA/VVWVVVWW^ 



Fig. 301. — Measurement of Galvanometer Resistance. (Kelvin's.) 



That is, by putting a resistance of 5,000 to 10,000 ohms in 

 series with the cell and putting a shunt across both cell and 

 resistance of about 1 ohm. This device is much better than 

 shunting the cell alone which runs it down, and, moreover, the 

 resistance in series is not too high and within the range of the 

 usual resistance boxes. 



On closing the battery key a deflection is observed on the 

 galvanometer. If this is too large the aforesaid resistance and 

 shunt in the batterj'^ circuit must be readjusted to obtain a 

 suitable deflection. The galvanometer must not be shunted. 

 Then the balancing key is tapped and the resistance in the 

 third arm adjusted until the closing or opening of this key 



