during the process of Sheathing. 



173 



relay. The instant a fault either in insulation or continuity 

 occurs, the balance is disturbed and the needle closes the circuit 

 of another battery containing an alarm so as to give a signal. 



(2) The position of the resultant fault is independent of tem- 

 perature so long as the latter remains uniform — also of the length 

 of the cable and the time occupied in testing, i. e. of what is 

 called electrification — and, further, of the electromotive force. 

 Thus the position of the resultant fault may be calculated exactly 

 and tested as nearly as the sensibility of the present galvano- 

 meters permit. 



(3) By observing the position of the resultant fault every day 

 and finding its maximum resistance, we are able to determine 

 the position of a slight fault, which may afterwards occur, with 

 great exactness. 



As full proof of the existence of the above advantages is re- 

 quired, it will be necessary to enter into the theory of this me- 

 thod for testing cables; and this I shall endeavour to do as 

 briefly as possible in the following. 



Suppose, first, a cable to have only two faults, all other 

 points being insulated absolutely ; then by connecting such a 

 cable according to fig. 1, we have fig. 2. 



Fig. 2. 



The current of the battery E enters through m on one side, 

 and through o', o", representing the two faults, on the other. 



Fig 2 is more clearly represented by fig. 3, — a, b, c, d, g, f, h, 

 p, and q being the resistances of the different branches, and 

 the respective capitals the different intensities, before balance in 

 the galvanometer is established. These diagrams are evidentty 

 different from the one usually employed, where o' and o n fall 

 together; and the first investigation will therefore be, what 

 relation exists between the nine different resistances in case G=0, 



