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

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

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Measurement and Instrumentation
Potentiometer method of DC voltage measurement is more accurate than direct measurement using a voltmeter because

 It loads the circuit to maximum extent.
 It loads the circuit moderately.
 It uses centre zero galvanometer instead of voltmeter.
  It does not load the circuit at all.

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