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The substitution method gives a more accurate result than the plain ammeter-voltmeter method because

  • A.the standard resistance used in it is always of a higher grade than any instrument scale
  • B.the measurement is a null one and therefore independent of the sensitivity of the detector
  • C.the current through the unknown is very much smaller and self heating is thus avoided
  • D.the errors of the supply and the instrument cancel

Answer: D

Two readings are taken with the same supply and the same instrument, and the standard is adjusted until they agree. Any scale error or supply drift affects both readings alike and therefore drops out, leaving the accuracy of the standard as the limiting factor.

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