ELECTRICAL METHODS 



543 



circuit of the Megger, and this flow of current causes a potential drop due to electrode 

 and circuit resistances. Corrections for these resistances are made by calibration. 

 For one instrument, the following relationships were obtained : 



TABLE 19 

 CALIBRATION DATA FOR MEGGER INSTRUMENT 



Normal resistance of potential circuit includes calibration resistance. Calibration 

 resistance is the resistance assumed for the electrode stakes Pi and P2 in calibrating 

 the instrument. Permissible variation means that the total resistance of the two 

 electrodes must lie within the figures given for the error in the reading not to exceed 

 the percentage indicated. 



The procedure for correcting the electrode resistance is as follows: 

 Pi and P2 = actual resistances at potential stakes. 

 Actual resistance in potential circuit = N + Pi + P2 — C. 

 If Pi + P« = C, the reading is correct. 

 If Pi + Pa is less than C, the reading is too high. 

 If Pi + P2 is greater than C, the reading is too low. 

 N + Pi + P2—C ^ 



The correction factor is 



N 



True resistance = Observed reading x F 



F— 1 

 % error = — ;; — x 100 



Finally the corrected values of the resistance E/I may be changed to apparent resis- 

 tivity values by the various formulas previously derived for the different electrode 

 configurations wherein two current stakes and two potential electrodes are used. 



