Measurement and Instrumentation
If an inductance L is connected in one arm of bridge and resistance R1, R2, R3 in other three arms

 The bridge can not be balanced.
 The bridge is balanced for some specified value of R1, R2, R3.
 The bridge can be balanced.
 The bridge is balanced for some specified value of frequency.

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

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