474 SUBMARINE CABLE LAYING AND REPAIRING. 



farad each oq a high-resistauce fault to 10 microfarads ou a 

 low one. The slides are the usual form divisible into 10,000 

 equal parts. 



On shore the testing battery is put on (zinc to line) through 

 the resistance R for a pre-arranged number of minutes. This 

 makes the potential p.^ at the ship end negative. The battery 

 on the slides is connected carbon to slides and zinc to earth, 

 thus giving a positive potential to the travelling contact. The 

 charges given simultaneously to condensers Fj and F2, on pres- 

 sing down the mixing key, are, therefore, of opposite sign. If, 

 now, the condensers are of equal capacity, and there is no deflec- 

 tion on the galvanometer after the mixing key has been raised 

 and the charges mixed, it follows that they have both been 

 charged to the same potential, or in other words that the poten- 

 tial on the slides at the travelling contact is equal to the 

 potential at the end of the cable. Should the capacities Fj 

 and F3 of the condensers not be exactly equal, the slide reading 

 (after balance is obtained on the galvanometer) must be mul- 

 tiplied by the ratio 



We have, therefore, the potential p^ expressed as a slide read- 

 ing. The potential measurements at both ends must be reduced 

 to the same units, and the simplest way to do this is to express 

 them in volts. For this purpose, so far as the slide reading on 

 the ship Is concerned, it is only necessary to compare it with 

 a similar observation on the slides with a standard cell to 

 convert the slide reading to volts. For example, say the slide 

 reading is 950 divisions with a standard cell of r45 volts, and 

 3,603 when balanced against the potential ^j. Then we have 



jPi=r45 X ' =5-5 volts, 

 you 



The balance key is shown separately in the diagram for clear- 

 ness, but it usually forms part of the mixing key. A plug is 

 shown for putting the battery on or off, but this may be a key 

 or switch as convenient. 



When condenser discharge throws are taken on shore to 

 measure the potentials P and p it is advisable to make the 

 added resistance about the same as resistance x of cable up to 

 fault plus the resistance of the fault. This makes sufficient 



