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

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

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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 the magnitude of the quantity being measured.
 only phase of the quantity being measured.

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