206 ELECTRICAL MEASUREMENTS 



determine this point. To do this, wire up exactly as for a test, 

 close Ki, then disconnect the lead from the cable and leave it 

 hanging free. Be sure not to introduce a new source of error 

 by the arrangement for supporting the free end. Now throw 

 on the battery. If the e.m.f. be high the galvanometer will 

 probably give a slight deflection and then settle back to its 

 original reading. If it does this the deflection is due to the 

 electrostatic action incident to charging the apparatus to a high 

 potential. Note, with the key K 2 as shown, if there is a perma- 

 nent deflection of the galvanometer and its direction. Now 

 put Kz in the dotted position and adjust S for full sensitiveness 

 (m = 1), and again note the deflection if there be any. Leakage 

 may occur on the lead between its free end and the shunt; if 

 so, the deflection will be positive in direction and will not appear 

 until the key K% is placed in the dotted position. S should be 

 adjusted so that m = 1. If the leakage occurs on the leads from 

 the shunt box to the galvanometer or at the galvanometer 

 terminals, the deflection may be either positive or negative, as 

 follows: 

 Ground on left-hand lead 



Shunt adjusted for m = , D = + and small. 

 Shunt adjusted for m = 1, D = + and large. 



Ground on right-hand lead- 

 Shunt adjusted for m = oo , D = and large. 

 Shunt adjusted for m = 1, D and small. 



The test should be repeated with Ci reversed. 



When measuring X the key K 2 must not be thrown to the 

 dotted position until the cable is charged electrostatically. 

 Therefore, after closing Ki, allow at least 20 sec. to elapse be- 

 fore throwing K 2 to the right; this will prevent injury to the 

 galvanometer. 



Immersion. After the specimen has been immersed, sufficient 

 time must be allowed before the test is made for the cable to 

 become thoroughly saturated, for moisture to work into any 

 defects that may exist in the insulation, and for the insulation 

 to attain the temperature of the tank. In practice no tests 

 should be made until after 12 hours. 





