226 APPLIED SCIENCE 



dent the pipe got choked up instead of having a hole in it, 

 nothing would be easier than to tell where the obstruction lay, 

 by measuring the quantity of water we could pour into the 

 pipe before filling it. Then knowing the capacity per unit of 

 length, we could calculate the distance by simple division. 

 Exactly so the capacity per unit of length of an electric cable 

 for electricity can be, and is measured, so that if the conductor 

 is broken inside the insulating sheath, without a fault of insu- 

 lation occurring, the distance of such a fault can be obtained 

 by a simple measurement of the charge which the insulated 

 conductor will take. In short, we can measure current, resist- 

 ance, potential, and quantity. What is to be measured depends 

 on the nature of the fault observed ; but from these measure- 

 ments or some of them, wherever they can be made simul- 

 taneously at each end, the position of the fault can be fixed. 

 Unfortunately, no system of tests on one side of a fault can 

 give its position. A bad fault far off, and a small fault close 

 at hand, cause all the elements which can be observed to vary 

 simultaneously, so as to give no clue as to which has occurred. 

 A bad fault, or one with little resistance, can have its position 

 fixed on the assumption that it has no resistance ; but a slight 

 fault absolutely requires the distant test before its position 

 can, even approximately, be determined. Fortunately, signals 

 from the distant end can always be sent past such a fault. 

 We are now in a position to consider the tests hitherto used 

 during laying and the improvements used on the Atlantic 

 expedition. 



In very early days people were satisfied if they could speak 

 through a cable whilst it was being laid. Then came the 

 simple insulation test at definite times. Then more complex 

 tests, spaced off into five minutes of this, ten minutes of that, 

 and six minutes of the other, so that each hour was cut up into 

 complex fractions, during which the ship and shore had simul- 

 taneously to make more or less complicated changes. If a 

 fault was detected during one arrangement, perhaps half an 

 hour would elapse before the time for speaking and either 

 sending or receiving intelligence would come round. Or, worse 

 still, a fault might occur and not be detected because the con- 

 nections at the time were arranged for speaking, or for a mere 



