66 SUBMAEINE CABLE LAYING AND REPAIRING. 



If the diameter method of testing is employed, the deviation 

 scale is set with reference to the sliding contact through a 

 number of divisions appropriate to compensate for the small 

 difference of diameter between the standard and the test wires. 

 If the mass method is employed, this scale is set to compen- 

 sate for small differences in mass between the standard and the 

 test wires. These settings are in accordance with a simple 

 rule. 



Standard resistance coils of copper for any nominal size of 

 wire, representing any length and of 100 per cent, conductivity,, 

 may be used instead of a length of the actual wire. 



■VWVWA/ 1 



100 200 300 400 500 609 700 800 900 lODO 



I , I .. I I I r 



Slide Wire 



DC 



J 



%f 



"wr 



standard Test Wire 

 Wire 



■A^AAAAA/WWWV 



r\ rv£)Amperes 



Fig. 36. 



The theory of this instrument is very thoroughly explained 

 in the treatise on " The Conductometer and Electrical Con- 

 ductivity," by Eollo Appleyard. 



The potentiometer method is shown diagrammatically in 

 Fig. 36. The test wire is connected in series with a standard resis- 

 tance, one or two secondary cells and an ammeter. A resistance 

 capable of fine adjustment may be introduced for the purpose^ 

 of regulating the current to any required strength, say two or 

 three amperes, and to keep it constant. The resistances are 

 then proportional to the voltage at their respective terminals^ 

 and the relation between the voltages on each resistance is- 

 obtained on the slide wire. A constant potential is maintained 



