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
A Manganin swap resistance is connected in series with a moving coil ammeter consisting of a milli ammeter and a suitable shunt in order to

 Minimize the effect of stray magnetic fields.
  Minimize the effect of temperature variation.
 Reduce the size of the meter.
 Obtaine large deflecting torque.

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Measurement and Instrumentation
A resistance is measured by the voltmeter-ammeter method employing dc excitation and a voltmeter of very high resistance connected directly across the unknown resistance. If the voltmeter and ammeter reading are subject to maximum possible errors of ± 2.4 % and ± 1.0 % respectively, then the magnitude of the maximum possible percentage error in the value of resistance deduced from the measurement is nearly

 ± 3.4%
 ± 1.4%
 ± 2.4%
 ± 1%

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