472 SUBMARINE CABLE LAYING ANB BEPAIBING. 



make the single measurement on board and the other two on 

 shore, where they could be made with greater convenience 

 and exactness. Now, however, null methods have been devised 

 for all three measurements, and they can be made as easily on 

 shipboard as on shore, and very accurately. Consequently the 

 test can be reversed, either end freeing in turn and taking the 

 single potential while the other takes the two potentials. This 

 test is therefore brought in line with those most useful in 

 localising partial earth faults. 



The theory and connections of the Clark test may be briefly 

 stated as follows. A battery aad known reaiatancaR(Fig. 259) 

 are connected to the end A of a cable the other end B of which 

 is freed. The potential at P falls proportionately to i) at the 

 junction of the resistance and cable and to pi at the fault C. 



B. 



Mill' JK' 



I 



B 



1 End free 



Fault 

 Fi«. 259.— Clark's Fall of Potential Test. 



The fall from this point to zero potential takes place over the 

 resistance of the fault itself and the return circuit, but there 

 is no fall over the remaining part, CB, of the cable, because the 

 end at B is insulated. The potentials P and^ are measured at 

 one end and ;p-^ at the other. We then have, by Ohm's law, 



X _p -pi 

 K~P-io 

 whence the resistance to the fault from the A end is 



a;=RgJ:^i ohms 



P— 7> 

 It is sometimes useful to see what the fault resistance is, and 

 this can be found by 



f=Ji^J'^^-x^ R 



P-2J -' ^"T^-p 



