THE LOCALISATION OF BEEAKS AND FAULTS. 405 



The two-current Kennelly test entirely eliminates this possible 

 error by the balances being taken to false zero. When 

 readings are to fahe zero, the apparent resistances due to 

 polarisation and earth current do not appear in the formula, 

 as they are eliminated in the balances. 



Mr. Herbert E. Cann having carried out a large number of 

 tests on artificial exposures of various areas (Electrical Bevierv, 

 Dec. 18, 1896), found that the Kennelly ratios as above gave 

 results generally too high, and that the ratio 4:2:1 gave more 

 accurate results. 



With the ratio 4:2:1 the formula is also considerably 

 simplified — namely, distance of break becomes 



pj = C + B-A, 



where A is the balance with lowest current c, 



B ,, ., ,, intermediate current 2c, 



C „ ,, ,, highest current 4e, 



This ratio, of course, requires less total battery power than 

 with the Kennelly ratios, and in this respect is more convenient. 

 The readings should be taken in the order C first, B next, 

 A last. 



The highest testing current (4c) should not exceed 25 milli- 

 amperes to line with a high resistance break, the best results 

 being obtained with about 20. If the current much exceeds 

 this a considerable quantity of gas is set free at the fault, 

 which in small exposures causes unsteady and unreliable 

 results. On the other hand, if the testing current be very 

 weak slight changes in the earth current influence the result. 

 For this reason the lowest current (c) should not be less than 

 4 or 5 milliamperes irrespective of the surface of exposure. 



Rymer-Jones Two-Current Break Test to True Zero. — On 



a large number of tests on artificial exposures (Electrical 

 Review, Jan. 1, 1897) Mr. J. Rymer-Jones describes having 

 found that the three-current test could be further simplified 

 by taking only two balances to true zero. Using the 

 Cann ratios and formula, and taking the balance C with the 

 highest current (4c) first, he found that over a wide range of 



