Measurement and Instrumentation
For power measurement of three phase circuit by two wattmeter method, when the value of power factor is less than 0.5 lagging

Pressure coil of the wattmeter will become ineffective.
Both give the same readings
One of the wattmeters will read zero
One of the wattmeter connections will have to be reversed

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

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