Measurement and Instrumentation
The resistance offered to the passage of an in-varying current by a column of mercury at the temperature of melting ice of mass 14.4521 gms of uniform cross-sectional area and length 106.3cm is

One ohm according to working standards
One ohm according to SI units
Standard ohm
One international ohm

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Measurement and Instrumentation
In DC potentiometer measurments, a second reading is often taken after reversing the polarities of the DC supply and the unknown voltage and the average of the two reading is taken. This is with a view of eliminate the effects of

 Ripples of the DC supply.
  Stray thermal emfs.
 Errorneous standardisation.
 Stray magnetic fields.

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