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

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

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Measurement and Instrumentation
The measurement of a quantity

  is a comparison of an unknown quantity with standard.
 is a comparison of an unknown quantity with another quantity.
 is a comparison of an unknown quantity with known quantity.
 none of these.

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