448 



SUBMARINE CABLE LAYING AND EEPAIKIN&, 



where x = distance to fault from the testing station, E = added 

 resistance to balance, and L = true CE. of looped cables 

 (metallic circuit). 



Correction for Metallic Circuit. — If the insulation of the 

 cables is low there will be leakage between cables, which will 

 have the effect of shunting the CR and giving a result too 

 low. The leakage is from cable to cable, and may generally 

 be taken as approximately uniform along the length of each. 

 To obtain a correction the loop connection at the distant sta- 

 tions should be opened and the two cable ends freed. The 

 insulation resistance between conductors after one minute's 

 electrification is then measured on the bridge, with no earth 

 on the key or bridge, as in Fig. 250. 



Earth. 



Fig. 251. — Varley Loop. 



In Mr. J. Elton Young's able and valuable work " Electrical 

 Testing for Telegraph Engineers," the subject of corrections 

 for CR and insulation is very thoroughly gone into (pp. 95 to 

 104) and the true meaning and applicability of Hockin's and 

 Schwendler's formulae fully explained. Mr. Young recommends 

 Hockin's correction for the metallic loop where both cables are 

 imperfectly insulated and the distribution of leakage is 

 approximately uniform. 



Hockin's formula XIV. given in his original Paper on 

 " The Corrections to be Applied to the Apparent Resistance of 

 the Conductor and Insulator of a Telegraph Line when deter- 

 mined in the usual way by Wheatstone Bridge " {Journal 



