THE LOCALISATION OF BREAKS AND FAULTS. 413 



50 milliampere range, it must not be forgotten to multiply or 

 divide the readings by the proper constant in each case^ so that 

 the current is expressed in milliamperes. 



The earth current potential may change during a test, and 

 it is therefore advisable to observe this again after the balances 

 have been taken. Before observing the earth current, the 

 cable should be put to earth for one minute to dissipate the 

 break potential set up by the testing current. If the readings 

 show a definite change at any point and the earth current has 

 also altered during the test the corrections corresponding to the 

 earth currents at the beginning and end of the test should 

 be worked out and applied to the respective sets of results 

 affected. When half a dozen good pairs have been secured, 

 it is as well to check the results by taking a second set with 

 different battery power and ratio. 



Before taking this or any other test it is very important to 

 overhaul the battery. If Leclanche's, see that the jars have 

 sufficient liquid in and a little surplus of sal ammoniac at the 

 bottom. All cells should be tested for E.M.F,, the exterior of 

 the jars dried, and the cells mounted on an insulated stand or 

 tray. Many weird earth currents and balances have been 

 traced to bad contacts or leakages in the cells or the inability 

 of the battery to sustain a current. 



The following example of a Schaefer localisation taken by 

 the author on the Sierra Leone-Accra cable will show the 

 whole observations from beginning to end and the best way of 

 setting down the results for working them out. 



Constant. — Standard cell (1-46 volts = 1,460 millivolts) 

 through suitable high resistance gave 268 deg. to Left. 



Earth Current — 

 Bafore commencing to test, 200^ to the right = 1,090 m/ volts. 

 One minute after testing, 260^ „ =1,417 ,, 



The working out of this is as follows : — 



If 268 divisions = 1,460 m/volts, 



T ,, . . 1,460 _ ,. 

 1 division =-^ = o-45. 



268 



Hence 200= = 5-45 x 200 = 1,090 m/volts, 

 and 260° = 5-45 x 260 = 1,417 „ 



