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.

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Measurement and Instrumentation
A Manganin swap resistance is connected in series with a moving coil ammeter consisting of a milli ammeter and a suitable shunt in order to

  Minimize the effect of temperature variation.
 Reduce the size of the meter.
 Minimize the effect of stray magnetic fields.
 Obtaine large deflecting torque.

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