Measurement and Instrumentation
The current coil of a wattmeter is connected to the CT of R-phase. The potential coil is connected across Y and B phases. The wattmeter measures

Power proportional to 3 phase power if the load is balanced
Active power in R phase
Active power of Y phase
Reactive power of R phase

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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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