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

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Measurement and Instrumentation
In measurement system, the function of the signal manipulating elements is to

 to perform non liner operation like clipping filtering, chopping and clamping.
 to perform linear operation like addition and multiplication.
 change the magnitude of the input signal while retaining its identity.
 change the quantity under measurement to an analogues signal.

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