TRANSACTIONS OF THE SECTIONS. 253 



4th. See how many cells will give the same deflection when only the galvanometer 

 is in circuit. 



5th. Repeat 2nd, 3rd, and 4th with various powers, aud take the mean of many 



results. 



The following formula will then give the resistance of the conductor : — 



Resistance of galvanometer r - 



Number of cells required in operation No. 3 ... n. 



Number of cells when galvanometer only is in circuit «'. 



Resistance of cable #• 



Then 



L±f=il,andtf=^-r. 

 n n' w 



With moderate care this will indicate the resistance to within 2 or 3 per cent. Care 

 must be taken that the earth-plate used, and also the battery-cells, offer no appre- 

 ciable resistance. 



Resistance coils and a differential galvanometer are much more exact, and should 

 always be used if possible. The author's standard, which has been adopted by the 

 Electric and International Telegraph Company, the Atlantic and other Telegraph 

 Companies, consists of the following units : 1, 2, 3, 5, 10, 20, 30, 50, 100, 200. These 

 allow of the coils being checked by themselves; thus 1+2=3, 2+3 = 5, 2+3 + 5=10, 

 &c., which is very useful in practice. 



Powerful currents must not be allowed to flow long through the coils, because they 

 are thereby warmed and their resistance increased. 



Case 2.— When (as is almost always the case) the fault itself offers resistance, but 

 the conductor is otherwise perfect, one of the two following methods will indicate 

 with sufficient precision the amount of resistance due to the conductor between the 

 operator and the fault, and also that of the fault, the former being the distance of 

 the defect : — 



1st. Have the conductor disconnected at the distant end (B), and 



measure the resistance = B 



This is the resistance of the conductor between the operator's 

 end (at A) and the fault, plus that of the fault (x+z). 



2nd. Have the conductor " put to earth " at B, when the current 

 on arriving at the fault, will split. Measure the resistance 



now(*+^) =r 



3rd. The resistance of the conductor alone when perfect . ... =S 

 Calling x the distance or resistance of the cable between the operator and the fault, 

 „ y the resistance of the cable between the fault and B, and 

 „ z the resistance of the fault itself, 

 we have 



This operation should, if possible, be repeated at the end B, which will indicate 

 the possible amount of error. 



Plan No. 2 requires that there be at each end galvanometers of known resistance, 



