404 SUBMARINE CABLE LAYING AND REPAIRING. 



During the test the key is clamped down zinc to line. It is 

 better to take the balance with the highest current first — that 

 is, balance C first, B next and A last in each set. This order 

 was recommended by Mr. H. E. Cann (Electrical Review, 

 Dec. 18, 1896) so that any cleaning of the exposure should 

 take place first with the high current, and the readings thus 

 be taken under a uniform condition of the break. 



Ti^is test does not take into account the change in the polar- 

 isation potential at the break due to variation of the testing 

 current. In the diagram (Fig. 238) the horizontal lines, as 

 divided up, represent respectively the resistance of the line 

 X, the break exposure /, the apparent resistance of the break 



E 

 due to polarisation — and the apparent resistance due to earth 



current -. The first line represents the balance by current c, 

 the second by current nc and the third by mc. The test being 



Fig. 238. — Analysis of Kennelly's Three-cxirrent Test to True Zero. 



to true or scale zero, the whole of these resistances are taken 

 account of in the bridge balances, and are therefore shown in 

 the diagram as parts of the same line, the lengths of the 

 respective lines indicating the approximate variation in resist- 

 ance with change of current. 



If the E.M.F.s of polarisation and earth current (E and e in 

 the diagram) remain constant when the testing current is in- 

 creased to nc and vie, the apparent resistances due to them are 

 eliminated in the formula for :*;, and this is what is assumed 

 to be the case in this test. But Mr. Schaefer's researches 

 show that E becomes E ^'\ln under a testing current of n times 

 the strength, and E ^'IJm with a current of m times the strength. 

 Hence the assumption that E remains constant leads to a 

 alight error, depending on the multiples of current employed. 



