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 magnetic fields.
  Stray thermal emfs.
 Errorneous standardisation.

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Measurement and Instrumentation
The measurment of very low and very high frequencies is invariably done using a frequency counter in which one of the following?

 Frequency measurment modes only.
 Frequency and period measurment modes, respectively.
  Period and frequency measurement modes respectively.
 Period measurment modes only.

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