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.
 Errorneous standardisation.
 Stray magnetic fields.
  Stray thermal emfs.

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Measurement and Instrumentation
It can be stated that

No generalisation can be made with regard to the flux densities in CT and PT
CT operates at almost the same flux density as a PT
CT operates with considerably lower flux density than a PT
CT operates with a higher flux density than PT

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