THE LOCALISATION OF BREAKS AND FAULTS. -iOl 



cally raising the charge key (that is, making the earthing period 

 as small as possible) if required; but if a longer period is desired 

 the charge key is first raised by hand and the discharge key 

 closed after the required interval. It is not necessary with this 

 key to move the slides index to the zero end for the absorption 

 discharge d as the discharge key has an earth contact for the 

 purpose. Tlie equivalent throw d-^ from the slides is then 

 obtained by putting the index at a position p-^ a little way 

 from zero and closing the discharge key. The working out of 

 the result is as previously described. 



Mr. Arthur Dearlove, in an able Paper on this test and its 

 correction {The Electrician, March 6, 1891, p. 537) as applied 

 to long cables, recommends an earthing period of three to four 

 seconds for cables of from 1,000 to 1,500 nauts length before 

 taking the absorption discharge, and this is usually followed. 

 He showed that the earthing period had an important influence 

 on the accuracy of the correction : if insufficient time was 

 allowed the cable was not properly discharged, and the correc- 

 tion became too large, making the apparent capacity too high. 

 The tests were taken on laid cables of 1,331 and 100 nauts 

 respectively, and the true capacity per naut was taken as that 

 determined on two-naut lengths at the factory, at a tempera- 

 ture of 75°F. The cables were each balanced against conden- 

 sers of 20 and 60 microfarads capacity. Oa the large cable the 

 mean of the tests gave an excess of only four-fifths of 1 per 

 cent, over the true capacity as measured in factory lengths. 

 This on a cable of 472 microfarads is a good proof of the use- 

 fulness of the Muirhead correction. The earthing period 

 allowed was three seconds, and it was found that a less period 

 complicated matters, as the absorption discharge was then first 

 in one direction and then in the other. lu such cases the 

 difference only was taken as the correction to apply, but the 

 author of the Paper poiats out that this effect may be used as 

 a means of finding the right time necessary for the "earthing 

 period " for any length of cable. For the purpose of verifying 

 the accurateness of the correction, tha periods of charge were 

 varied from 5 to 60 seconds, and it was found that the correc- 

 tion became somewhat too powerful with periods exceeding 30 

 seconds, but that varying periods of charge up to 30 seconds 

 did not lessen the accuracy of the correction. 



