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


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