368 SUBMAKINE CABLE LAYING AND EEPAIKING. 



false zero mark, the bridge is balanced, and the resistance of 

 the cable so determined is the correct resistance, the inter- 

 ference of polarisation and earth currents being eliminated. The 

 discharge from a cable at the moment of breaking the circuit> 

 of the testing battery makes it difficult at first to know when 

 to read the false zero, but it is less difficult when the fault or 

 break is a long distance off than when it is near. False zera 

 work is also rendered a much simpler operation by the use of 

 the Sullivan dead-beat galvanometer. 



The deflection is a decreasing one, due to both discharge and 

 polarisation ; but the discharge at its commencement is at much 

 higher potential than the polarisation current. On the other 

 hand, it falls more rapidly, there being one instant of time when 

 the potential of the discharge at the galvanometer terminal 

 coincides with that of the polarisation. That point is reached 

 in from one to three seconds, according to the length of the 

 cable up to the fault, and it requires considerable practice and 

 judgment to catch it at that point. False zero is also spoken 

 of as "cable zero," or "natural zero," or "inferred zero," as 

 distinguished from " instrument," " scale," or true zero. 



Polarisation of Fault or Break. — A little examination into 

 the action called "Polarisation" will repay attention. The 

 effect can be watched closely by an arrangement such as that 

 shown in Fig. 218. 



Into a glass beaker filled with sea water is put a small piece 

 of gutta-percha covered wire, W, the immersed end being bared 

 and exposing some of the copper wire. Opposite to this in the 

 water is a piece of galvanised iron wire, E, twisted to afford a 

 good surface. The two pieces are held steady by bending their 

 outside ends close to the glass and putting rubber bands round 

 to hold them. 



On connecting up five Leclanche cells with the zinc pole to 

 the copper wire, a stream of bubbles of hydrogen is observed 

 to be given off from the end of copper exposed. We may 

 regard the piece of insulated wire as part of a cable, and the 

 exposed bit of copper the fault or break, while the iron takes 

 the place of the sheathing wire of the cable. It is very good 

 practice to use such an artificial fault with a resistance of, say, 

 1,000 ohms in circuit, and then connect up to a bridge and 



