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.
 Stray magnetic fields.
 Errorneous standardisation.

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Measurement and Instrumentation
Assertion (A) : It is always desirable to take the reading of an indicating instrument very close to the full scale reading. Reason (R) : Accuracy of an indicating instrument is maximum at the full scale deflection and error increases as reading comes closer to the beginning of the scale.

 A is false bur R is true
 Both A and R are true and R is the correct explanation of A
 A is true but R is false
 Both A and R are true but R is NOT the correct explanation of A

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