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A moving iron type ammeter has few turns of thick wire so that
Resistance is less
Sensitivity is high
Damping is effective
Scale is large
Resistance is less
A moving iron ammeter is connected in series with the load to measure current, meaning it must have negligible internal resistance to avoid affecting the circuit current. By using a few turns of thick wire, the meter achieves a low resistance while maintaining sufficient Ampere-turns to produce the required deflecting torque.
A moving iron ammeter is connected in series with the load to measure current, meaning it must have negligible internal resistance to avoid affecting the circuit current. By using a few turns of thick wire, the meter achieves a low resistance while maintaining sufficient Ampere-turns to produce the required deflecting torque.
TdтАЛ=21тАЛI2d╬╕dLтАЛ тАФ Deflecting torque based on rate of change of inductance
R=╧БAlтАЛ тАФ Relation between resistance, cross-sectional area, and length
The deflecting torque in a moving iron instrument is given by TdтАЛ=21тАЛI2d╬╕dLтАЛ. To measure current without altering the circuit conditions, the instrument must act as a near-perfect short circuit. Thick wire reduces the DC resistance R=╧БAlтАЛ, where area A is large, keeping power dissipation I2R minimal and preventing excessive voltage drops across the meter.
Moving iron ammeters are always connected in series with the load.
Thick wire decreases resistance, minimizing the voltage drop across the instrument.
Few turns are used because the current to be measured is large, providing sufficient ampere-turns (NI).
High resistance would cause significant power loss and inaccuracies in circuit operation.
Cheap and robust construction.
Can be used for both AC and DC measurements.
Less susceptible to stray magnetic fields compared to PMMC.
Non-linear scale (cramped at the beginning).
Frequency errors due to changes in reactance.
Hysteresis and eddy current errors.
Industrial grade AC ammeters and voltmeters.
Switchboard instrumentation.
If the wire were thin, the resistance would be high, leading to a significant voltage drop and potential overheating of the instrument coil.
Option B (Sensitivity) is incorrect because sensitivity is typically defined as Ohms per Volt for voltmeters, not a primary objective for ammeter design.
Option C (Damping) is usually achieved via air friction using a vane attached to the moving system.
A is correct тАФ A moving iron ammeter requires very low internal resistance to ensure minimal interference with the circuit current when connected in series.
Always remember: Ammeters should have near-zero resistance (connected in series), while Voltmeters should have near-infinite resistance (connected in parallel).
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