Measurement and Instrumentation
To measure an A. C. voltage by using an A.C. potentiometer, it is desirable that the supply for the potentiometer in taken

From a battery
From the same source as the unknown voltage
From a source which is not the same as the unknown voltage
Any of the above

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Measurement and Instrumentation
In DC potentiometer measurments, a second reading is often taken after reversing the polarities of the DC supply and the unknown voltage and the average of the two reading is taken. This is with a view of eliminate the effects of

 Ripples of the DC supply.
 Errorneous standardisation.
  Stray thermal emfs.
 Stray magnetic fields.

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