THE LOCALISATION OF BREAKS AND FAULTS. 501 



•end nearest to the primary coil. This effect is more marked 

 the greater the number of turns in the secondary. 



Also, assuming the same terminal of the galvaaometer (say 

 the left-hand terminal) connected in turn to the ends of the 

 secondary coil (the right-hand terminal being to earth) as at B 

 and C, Fig. 278, the induced charge will produce swings in oppo- 

 site directions on the galvanometer. The end of the coil 

 which lies nearest to the primary will then be distinguished by 

 the larger swing on the galvanometer, and vice versa. 



The induced current depends upon the battery power used 

 in the primary, the sensitiveness of the galvanometer in the 

 secondary and the number of turns or flakes in the coils under 

 test. With large coils a shunt may be necessary on the gal- 

 vanometer, and with short coils larger battery power may be 

 required, and the galvanometer in its most sensitive state to 

 obtain convenient throws. 



Treatment of Cable Ends to Prevent Surface Leakage. — 



When cable is tested for insulation or capacity precautions must 

 be taken against leakage along the surface of the core from 

 conductor to earth at both ends of the line. In damp and 

 misty weather, or in tropical rainy seasons, surfaces are in a 

 state of moisture and sweat, and if cable ends, especially in 

 tank, are not carefully cleaned and dried for D.R. tests the 

 electrification may be unsteady, and insulation appear low, due 

 to no weakness in the cable, but to surface leakage at cable 

 ends or some part of the testing apparatus. 



The core should be exposed clear of jute yarns or taping for 

 at least 12 in. and rubbed over with a clean rag, containing a 

 little spirit or wood naphtha, to thoroughly clean the surface. 

 The end of the percha should be neatly trimmed to a long 

 taper, and the surface dried by a spirit lamp flame worked 

 lightly round so as not to melt the percha. Where the 

 surrounding air is moist, as in a tank shed or in wet weather, 

 the end is usually protected by a coating of paraffin wax, 

 melted and set round it in the manner shown in Fig. 279. 



Mr. W. A. Price has devised the very simple and ingenious 

 arrangement of a guard wire to prevent surface leakage at 

 cable ends (Electrical Pievieiv, December 6, 1895, p. 702). 

 This is a thin copper wire connected at one end between 



