THK LOCALISATION OF BREAKS AND FAULTS. -421 



Similarly the second pair : — 



Ai = 8,040-1, 308 = 6,732, 

 Bi= 7,190- 5iO=.6,680, 

 a; = 6,732-(o2x 1-943) =6,631 ohms. 



And the last pair : — 



Ai = 8,200- 1,308 = 6,892, 

 Bi=7,350-510=6,840, 

 A- =6,892 -(52x1 -943) = 6,791 ohms. 

 Mean of the three sets (as before) = 6,730 ohms. 



The Schaefer test is specially useful in localisations on board 

 ship where the break is not far away. In that case earth 

 currents are generally small and negligible, or can be rendered 

 so by increase of battery power and the correction term dis- 

 pensed with. This shortens the test and calculations consider- 

 ably and saves much time when time is especially valuable. 

 The distance to break in ohms is then simply 



a; = A-(A-B)P. 



Balances being to scale zero in this test is another great 

 advantage because tests can be quickly and accurately made 

 and greater appreciation obtained than is possible when work- 

 ing to false zero, the instrument being in its most sensitive 

 state. 



Also, being a two-current test, it obviously requires less 

 difference between the maximum and minimum testing currents 

 than a three-current Kennelly, and has, therefore, wider limits 

 in the possible currents which may be used. 



Lumsden's Method. — This test, due to Mr. Lumsden and 

 known originally as the polar test, is applicable when there is a 

 comparatively large area of exposure at the break or fault. 

 Polarisation then is small in amount and slow in action, and the 

 ordinary bridge balance can be satisfactorily obtained, using 

 both directions of current — that is, zinc continuously and carbon 

 for the short periods necessary. The negative or zinc current 

 cleans the fault and lowers its resistance, while the carbon or 

 positive current polarises it, or seals it up by a deposit of 

 chloride of copper, which raises the resistance. 



