THE LOCALISATION OF BEEAKS AND FAULTS. 



497 



charge (blades going to left) and to left (as in the diagram) for 

 discharge. For insulation tests key B is put with both handles to 

 left (blades to right), thus connecting cable to key A. The read- 

 ings are taken by putting handles of key A to left (as in the 

 diagram) for cable and to right for earth readings. Limit stops 

 are provided so that the two blades cannot come in contact with 

 each other. 



Tests of Cable in Tank. — Cables coiled in a tank possess con- 

 siderable inductance under test — that is, a magnetic field is set 

 up around the coils upon a current being sent through the cable, 

 which retards the starting, stopping or any change in strength 



— 6MMITyS 



Fig. 276. — Combination Keys for Capacity and Insulation. 



of that current. This field is set up in all straight wires and 

 cables through which currents are passed ; but, as is well known, 

 the magnetic eff'ect is inappreciable. When the cable is coiled, 

 however, the magnetic field is intensified by every turn of the 

 cable, and also by the iron of the tank. In CR tests, when 

 first the current is applied, the resistance measured appears 

 higher than its true value, because the inductance E.M.F. 

 opposes the current like a resistance. As the current is kept 

 on the opposing E.M.F. gradually falls to zero, the current 

 rising the while to its full value. It is therefore necessary to 

 wait till the current has been on for a few seconds, especially 

 when testing long lengths, before finally adjusting the resistance 

 to balance. Even if the correct resistance has been unplugged 



K £ 



