172 Mr. L. Schwendler on Testing Telegraph Cables 



now that the cable be insulated equally at all points, and with a 

 conductor also of homogeneous resistance, the resultant fault 

 must evidently be in the middle, which would be indicated, in 

 case we have a = d, by W=0. 



In case a cable is not homogeneous, the resultant fault will 

 not generally be in the middle ; but knowing the order of the 

 different parts, and their insulation and conducting resistances 

 in the whole cable, and having besides a correct mathematical 

 expression for the position of the resultant fault, which formula 

 we shall develope afterwards, it will be possible to calculate this 

 position with great accuracy. Knowing now the position of the 

 resultant fault by a correct calculation as mentioned, and being 

 at the same time able to test for this position exactly, the follow- 

 ing rule may be set up as a necessary criterion for every electri- 

 cally normal cable, assuming equal temperature throughout its 

 length : — 



The calculated position of the resultant fault must coincide with 

 the tested position. 



But by the scrupulous care bestowed upon the single lengths 

 of core at the insulating-works, its manufacture has so much im- 

 proved that the resistances of insulation and conductor come so 

 nearly to a fixed standard at a certain temperature that we may 

 in practice suppose a cable to be homogeneous, and, the tempera- 

 ture being the same throughout, express the above rule thus : — 



A cable is electrically normal when the resultant fault lies ex- 

 actly in the middle of the conductor. 



It is true that this rule is necessary ; but the fulfilment of the 

 condition expressed does not give us entire security of the fault- 

 lessness of the cable — as, for instance, when two faults occur of 

 equal magnitude and equally distant from the middle, which 

 would evidently not affect the position of the resultant fault. 

 The probability of this happening, however, is so small that the 

 above rule will be sufficient in practice, especially when the pre- 

 sent methods of testing insulation directly are applied in addition, 

 and when an instrument which I have constructed for the pur- 

 pose is employed, which is constantly testing the insulation and 

 continuity of the cable during its sheathing. 



The above method of testing cables during their sheathing, 

 resting, as it does, on the test for homogeneity of the insulating 

 covering (that is, on determining the position of the resultant 

 fault by a simple balance in Wheatstone's diagram), has the fol- 

 lowing advantages : — 



(1) This method furnishes practicable means by which it is 

 possible to test the cables for insulation and continuity together 

 permanently during manufacture. For this purpose the galva- 

 nometer in Wheatstone's diagram is used simply as a needle 



