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

ANSWER DOWNLOAD EXAMIANS APP