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

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

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Measurement and Instrumentation
Assertion (A): A hot wire ammeter has a cramped scale. Reason (R): The heat is proportional to square of current.

 Both A and R are true and R is correct explanation of A.
 A is false and R is ture.
 Both A and R are true and R is not correct explanation of A.
 A is ture and R is false.

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