during the process of Sheathing. 



171 



It will hardly be necessary to show that a permanent test for 

 insulation during the hazardous process of sheathing would ne- 

 vertheless be of far greater importance than the mere test for 

 continuity. 



Being entrusted at the cable-works of Messrs. Siemens with 

 testing their cables, I began to search for a method which would 

 not only give the insulation of a cable during manufacture at a 

 glance, and be applicable by means of an automatic instrument 

 during the sheathing-process, but be also on a level with the 

 delicate instruments now employed for cable-testing. 



This method, which I have had in practical operation for some 

 time, is based on the well-known principle on which the deter- 

 mination of a fault depends when the two ends of a cable are at 

 hand — known as the loop method. The two ends of the cable 

 are connected with the ends of the galvanometer (forming two 

 sides of a Wheatstone's balance), and the battery put in circuit, 

 using the leakage of the whole cable as one entrance of the cur- 

 rent, and the point m, fig. 1, as the other, a and d representing 

 two branch resistances, and W an adjustable coil. The pole of 



Fi 2 . 1. 



the battery connected with m is insulated completely from the 

 earth. It is evident that, however the insulation of this cable be 

 distributed throughout its length, there will always be for two 

 given branch resistances a and d a fixed value of W, for which a 

 balance in the galvanometer-circuit is established ; and the mag- 

 nitude of + W* indicates the position of that which we may call 

 in future the u resultant fault " i. e. the fault resulting from all 

 the partial faults of the w^hole cable. The resistance of this re- 

 sultant fault is the absolute insulation of the cable. Supposing 



* The minus value of W means, always, that W is to he placed at the other 

 end of the cable hefore a balance can be established. 



N2 



