406 SUBMARINE CABLE LAYING AND REPAIEING. 



current strengths and areas of exposures, the following relation 

 holds good : — 



^^ = 0-5576. 

 A-B 



Hence C = B - 0-5576 (A - B), 



from which it is evident that one balance, C, can be dis- 

 pensed with, as it can be calculated from the other two, 

 A aad B. The test is therefore made by taking two balances 

 only with currents in the ratio of 2 to 1 to true zero : — 



A with current c, 

 B „ „ 2c. 



The balance B with the higher current is taken first. By sub- 

 stitution of the above value of C in the Cann formula we have 

 for this test the distance of fault or break 



aj = 2-5576B-l-5576A. 



This formula is most correct,when using testing currents of 20 

 and 10 milliamperes. When the fault is of small exposure it is 

 better to use lower currents, say, 10 and 5 milliamperes, to 

 prevent the formation of gas at the fault. In that case a 

 resistance of 10 ohms should be added to the result to make it 

 agree with a 20:10 milliamperes test. 



Schaefer's Break Test to True Zero.— In 1897 Mr. C. W. 

 Schaefer published hia investigations and laws connecting the 

 Variation of the potential and resistance of a break with the 

 strength of testing current (The Electrician, Oct. 15, 1897). In 

 Kennelly's three-current test the resistance effect of the polari- 

 sation potential (together with that of the earth current) was 

 eliminated on the assumption that it remained constant for 

 changes in the strength of the testing current. Mr. Schaefer 

 found that this potential varied in the direct proportion of the 

 4"3th root of the current. He further considered the break 

 resistance as the combined effect of the exposure and polarisa- 

 tion resistances. 



Dr. Kennelly's researches established the law of variation 

 in resistance of the exposed conductor at a break to be in the 

 inverse ratio of the square root of the testing current. Mr. 

 Schaefer's researches established the law of variation in the 



