Compare PMMC & MI instrument.
Points
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PMMC instruments
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MI instruments
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Principle
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When current carrying conductor is placed in a magnetic field, it experiences mechanical force
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Piece of iron is attracted /repelled by magnet or magnetic field.
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Scale
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Uniform
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Non-uniform
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Torque/Weight ratio
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Higher
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Lower
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Application
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PMMC instruments are used only for DC measurements
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Used for DC as well as AC measurements
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Cost
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Higher cost for same range
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Lower cost for same range
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Damping
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Eddy current
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Air friction
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Sensitivity
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More sensitive
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Comparatively less sensitive
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MI Instrument
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PMMC Instrument
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It works on the principles of magnetism.
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It works on the principle of DC motor
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Deflection torque is proportional to square of current
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Deflection torque is proportional to current
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Damping is provided by air damping
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Damping is provided by eddy current
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Controlling torque is proportional to Sine
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Controlling torque is proportional to angle of defection
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Gravity controlled instruments.
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Spring controlled instruments
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Non uniform scale.
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Scale is uniform
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Robust, reliable accurate.
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Delicate, sensitive and accurate
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Cheap.
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Costly.
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high power consumption than moving coil
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Low power consumption
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It is used both in A.C. and D.C. Circuits.
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It is used only in D.C. Circuits.
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Can be used as Ammeter, Voltmeter and Watt meter
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Can be used as voltmeter, Ammeter, Galvanometer, ohmmeter
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