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The reason a low resistance cannot be measured by the same arrangement as a medium one is that

  • A.the thermoelectric emf generated at the junctions rises in proportion to the resistance
  • B.contact and lead resistance become comparable with it
  • C.the current needed to produce a usable reading would melt the specimen under test
  • D.the insulation surrounding the specimen begins to carry an appreciable leakage current

Answer: B

A pair of screw terminals and a length of connecting wire contribute a few milliohms each. Against a 0.001 ohm specimen that is not a small correction but the greater part of the reading, so the potential connections must be brought out separately from the current ones.

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