50 THE SCIENTIFIC PAPERS OF 



ductor and the insulating covering by simple numerical expression 

 in units of resistance. 



The unit of resistance we have adopted is that of a column 

 of mercury 1 metre in length and of 1 square millimetre sectional 

 area, taken at the freezing point of water. The advantages of 

 this unit have been fully set forth by Mr. Werner Siemens in a 

 treatise published in " Poggendorff' s Annalen," vol. 110. 



In expressing the degrees of conductivity of both the wire and 

 the insulating medium in definite units of resistance we obtain 

 not only the advantage of a more accurate comparison between the 

 results of different indication, but subsequently when the separate 

 coils are united with a single cable, we have an admirable means 

 of judging its electrical condition if we compare the total resis- 

 tances of both the conductor and insulating medium with the 

 sum of the resistances previously obtained in testing each coil 

 separately, due allowance being made, of course, for change of 

 temperature. 



But the principal advantage derived from this system of 

 measuring consists in the facilities it affords in determining the 

 position of a fault in the cable while it is being laid and after 

 submersion. 



In carrying this, system into practice, we construct in the first, 

 place coils of definite resistance, which are capable of being 

 combined in such a manner that we can vary the total resistance 

 between the limits of 1 unit and 10,000. 



By inserting these alterable resistances into one branch of a 

 Wheatstone's bridge, the resistances of the copper or insulating 

 covering of a cable of considerable length can be ascertained. If, 

 however, it is required to ascertain resistances beyond the limits of 

 the resistance coils, we adopt another arrangement on the principle 

 of the Wheatstone's bridge, which consists in making the two> 

 permanent branches of the same also changeable. A, B, C, D, 

 represent the four branches of this arrangement, A, C, and B, D, 

 being in connexion with the galvanometer, A, B, and C, D, the 

 terminals of a battery (Plate 1, Fig. 1). 



No current will pass through the instrument when the rela- 



A C 



tion =fs- = -pr exists. But as in Wheatstone's arrangement A is 

 B D 



always equal to B, the unknown resistance D is equal to the re- 



