224 APPLIED SCIENCE 



can be measured with extreme accuracy, and that, as the resist- 

 ance is proportional to the length, the length in circuit can be 

 calculated from the resistance. Still, from our one measure- 

 ment, we have not got information enough to know certainly 

 where the fault is we only know that it cannot be more than 

 ten miles off ; it may be less, for the fault itself may have a 

 certain resistance, and about the fault we as yet know nothing. 

 But suppose we can now obtain a similar measurement from the 

 other end of the cable, and this gives 600 units, while the whole 

 length of the cable is 150 miles, we shall then know that the 

 fault is five miles from our end, and has a resistance equal to 20 

 units ; the resistance as measured from our end consists of five 

 miles of conductor and the fault, or 40 units in all, that from the 

 other end consists of 145 miles of cable and the same fault, or 

 600 units in all, and no other position or resistance of the fault 

 will agree with the two observations made. A comparison with 

 a pipe of water may make this clearer to non-scientific readers. 

 Let us take a pipe 150 yards long, and suppose that we know 

 exactly how much water will run through any given length of 

 a pipe of that diameter from given cisterns at each end. Now, 

 suppose a leak to occur in that pipe : if we stop up the far end 

 and let the water run in from our cistern, we find that as much 

 water runs out as would be allowed to pass by a pipe ten yards 

 long, we then stop up our end of the pipe and let water run in 

 from the far cistern. We find as much water is conveyed away as 

 would be allowed to pass by a pipe 150 yards long; then, as 

 in the electrical case, the leak in the pipe must clearly be five 

 yards from our end, and it must have a resistance equal to that 

 of five yards of pipe. Thus the position of a leak in a water- 

 pipe might be discovered, although the leak itself were buried 

 in the ground. The electrical experiment is quite analogous to 

 this, and is in practice made much more easily than the ex- 

 periment with water-pipes could be made, for the laws of the 

 flow of water in pipes are much less well understood, and less 

 simple than the laws of the flow of electricity, although we may 

 think we know better what water is than what electricity is. 



In cables containing more than one wire, the above test, or 

 something analogous to it, can always be made, for the faulty 

 and good wire being joined together at the distant station, can 



