THE LOCALISATION OF BREAKS AND FAULTS. 



411 



To be strictly correct in the above observations the high 

 ■resistance used with the galvanometer, when taking the stan- 

 dard cell deflection, should be increased by an amount equal 

 to the CR of cable in circuit less the resistance of the standard 

 <3ell itself. The latter is normally about 300 ohms, but may in 

 some cases b3 up to thousands of ohms when the cell is old. 

 When the CE, is high, say in the neighbourhood of 8,000 or 

 9,000 ohms, it becomes appreciable in comparison to 100,000 

 ohms, and the earth current deflection is less than the proper 

 equivalent of the potential by about 8 per cent. With any- 

 thing over 5,000 ohms CR it is better to use ^ or |^ megohm 

 for the high resistance in taking the constant. The error is 

 then inappreciable without the trouble of equalising the total 

 resistance in circuit in the two observations. 



High Resistance 



Cut-out 



Carbon 

 Fig. 243. — Connections for taking; Bridge Balances in Schaefer's Test. 



Bridge Balances. — Connections as in Fig. 243 with a two-way 

 switch to key for rapidly changing the number of cells. The 

 battery should be arranged to give currents to line in the ratio 

 of about 2 J to 1. It is not necessary or even desirable to spend 

 time getting this exactly, so long as the ratio is not less 

 than 2 to 1 or more than 3 to 1. A suitable number of 

 cells with 600 miles or so of cable is 37 and 16 Lelanches. 

 Below 12 cells there is too little appreciation and the line current 

 should never be less than 3 milliamperes. 



With switch first at high battery power clamp left hand 

 key down zinc to line and obtain balance B to scale zero, using 

 even bridge ratios. The galvanometer short-circuit key to be 

 tapped in the usual way as required to control the spot until 

 balance is obtained. 



