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 to maximum extent.
 It loads the circuit moderately.

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Measurement and Instrumentation
In AC circuits, the connection of measuring instruments cause loading effect errors which may effect

 both of above.
 magnitude, phase and waveform of the quantity being measured.
 only phase of the quantity being measured.
 only the magnitude of the quantity being measured.

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