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A high resistance measured by the direct deflection method reads lower than its true value on a damp day. The cause is

  • A.the sensitive galvanometer being affected by moisture in its suspension and bearings
  • B.the applied direct voltage falling because of leakage within the supply itself
  • C.surface leakage across the specimen
  • D.the capacitance of the specimen increasing with the humidity of the surrounding air

Answer: C

Moisture lays a conducting film over the surface, and the current along that path is added to the wanted volume current so the computed resistance comes out low. A guard electrode intercepts the surface current and returns it to the source without letting it reach the detector.

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