THE LOCALISATION OF BREAKS AND FAULTS. 525 



divided into ten equal parts, the potential at, say, the 

 point 8, is eight times that at point 1, twice that at point 4, 

 and so on. 



Without knowing the current flowing through the wire or its 

 resistance, we can, by means of a standard cell, find the poten- 

 tial at one point on the wire, from which the potential at all 

 other points can be found. 



The negative pole of the standard cell is connected to the 

 negative end B of the wire, and the positive pole to a galvano- 

 meter and key. A resistance of a few thousand ohms is put in 

 circuit with the standard cell to prevent any large current 

 being taken from it or put into it when the slide is in out-of- 

 balance positions. If the free end of the wire is now placed in 

 contact with the wire AB and moved along it (the key being 

 tapped) a position will be found where no current passes 

 through the galvanometer. 



The E.M.F. of the standard cell is then balanced by the 

 opposing potential in the wire, and therefore the potential at 

 this point is equal to that of the standard cell. Supposing 

 the balance obtained with the movable wire was at 7 on the 

 wire AB, with a standard cell of 1-4 volt; the potential along 

 the slide wire AB is then 

 7 

 1"4 X — = 2 volts per division. 



Now the standard cell may be replaced by any battery 

 whose E.M.F. is required. If the point of balance with the 

 battery is at 9, as in Fig. 296, the E.M.F. of that battery is 

 18 volts, and if at, say, 7 "75, the E.M.F. is 15| volts, and so on. 



It is not necessary to know the E.M.F. or resistance of the 

 main battery that produces the current in AB. It must, however, 

 have an E.M.F. greater than that of the test battery balanced 

 against it. 



The wire potentiometer cannot be used for more than 

 moderate voltages because the wire would have to be of higher 

 resistance, smaller diameter and longer length than practicable 

 in order to get sufficient range of potential. 



When a high-resistance potentiometer is necessary it must 

 be constructed of independent coils of wire connected in series. 

 But in any such series of coils there is not the fine adjustment 

 that can be got with a wire, and it is not practicable to increase 



