64 



SUBMARINE CABLE LAYING AND REPAIRING. 



percentage conductivity and of the same diameter and the same 

 weight. The resistance of the standard wire and test wire may 

 be balanced by the bridge method, using a slide wire for the 

 ratios, or by a slide potentiometer method. In Appleyard's con- 

 ductometer (Fig. 25), which is an instrument of the former 

 class, there is a bridge wire of platinum silver 2 metres in length. 

 Only a portion of this wire in the centre is required for 

 balancing, and the end portions are coiled up inside the instru- 

 ment. If the percentage conductivity of the standard wire 

 was exactly 100, and all test wires balanced against it were 



Fig. 35. — Appleyard's Conductometer. 



exactly the same diameter as the standard wire, the slide bridge 

 could be graduated to give the percentage conductivity of any 

 wire tested, according to the position of balance. But it is not 

 always possible to obtain a standard wire of exactly 100 per 

 cent, conductivity, and the diameters of wires in diflferent hanks, 

 although nominally the same gauge, vary within appreciable 

 limits. In this instrument a compensating arrangement is 

 adopted to correct the position of the sliding contact of the 

 bridge, giving it the position that it would have if the standard 

 wire were exactly 100 per cent, conductivity and if the 

 diameters of the samples tested were equal. 



