Moving Iron (MI) Instruments
• Generally used to measure AC current or
voltage (but can measure DC current and
voltage without any external circuit)
• Pointer is connected to specially designed
soft iron that moves according to the intensity
of magnetic field acting on it
• The magnetic field is produced by current
Classification of MI
1) Repulsion (or double iron) type
2) Attraction (or single iron) type
Types of torque in MI
• Deflecting torque produces movement on an
aluminum pointer over a calibrated scale
• Control torque: provided by spring or weight
• Damping torque: Pneumatic (mechanical
damping) aluminum vane attached to the shaft
Constructional details of
attraction type MI
• Moving element : Soft
• Coil: Produces magnetic
field & magnetize the iron
• Control springs or
weights, aluminum vane,
aluminum pointer, shaft,
calibrated scale, mirror, etc
Constructional details of Repulsion
• Two concentric iron vanes (Fixed & Movable)
• Solenoid coil (stationary)
• Pointer attached to movable vane
• Movable vane is curved rectangular in shape
• Fixed vane is tampered
• Controls springs, Aluminum pointer, Aluminum
vane, calibrated scale, mirror etc
• Current flow magnetizes the coil
• The two iron vanes become magnetized with north poles at
their upper ends and south poles at their lower ends for one
direction of current through the coil
• Due to repulsion, the unbalanced component of force,
tangent to the movable element, causes it to turn against the
force exerted by the springs
• When no current flows through the coil, the movable vane is
positioned so that it is opposite the larger portion of the
tapered fixed vane, and the scale reading is zero
•The amount of magnetization of the vanes depends on the
strength of the field, which, in turn, depends on the amount
of current flowing through the coil.
Design for MI Ammeter and MI
Ammeter: Coil with few turns of large
1)Solenoid with many turns of small wire
2)Portable size: a) self contained series
resistance b)ranges up to 750 V
3) Higher ranges are obtained by the use of
additional external multipliers
Ranges of Ammeters and Voltmeters
For a given moving-iron instrument the ampere-turns necessary
to produce full-scale deflection are constant. One can alter the
range of ammeters by providing a shunt coil with the moving coil.
Shunts and Multipliers for MI instruments:
For moving-iron ammeters: For the circuit shown in Fig.42.9,
let Rm and Lm are respectively the resistance and inductance
of the coil and Rsh and Lsh the corresponding values for shunt.
The ratio of currents in two parallel branches is
It is difficult to design a shunt with the appropriate inductance, and shunts are rarely
incorporated in moving iron ammeters. Thus the multiple ranges can effectively be
obtained by winding the instrument coil in sections which may be connected in series,
parallel or series-parallel combination which in turn changing the total ampere-turns in
the magnetizing coil.
For moving-iron voltmeters:
•Voltmeter range may be altered connecting a resistance in series
with the coil. Hence the same coil winding specification may be
employed for a number of ranges.
•Let us consider a high resistance Rse is connected in series with the
moving coil and it is shown below.
•Note: An ordinary arrangement with a non-inductive resistance in series with the
fixed coil – results in error that increases as the frequency increases. The change of
impedance of the instrument with change of frequency introduces error in signal
•In order to compensate the frequency error, the multiplier may be easily shunted by
• The instruments are suitable for use in a.c and d.c circuits.
• The instruments are robust, owing to the simple construction of the moving parts.
• The stationary parts of the instruments are also simple.
• Instrument is low cost compared to moving coil instrument.
• Torque/weight ration is high, thus less frictional error.
•Errors due to temperature variation.
•Errors due to friction is quite small as torque-weight ratio is high in moving-iron
•Stray/demagnitization fields cause relatively low values of magnetizing force produced
by the coil. Efficient magnetic screening/iron-case/thin shield over the working parts is
essential to reduce this effect.
•Error due to variation of frequency causes change of reactance of the coil and also
changes the eddy currents induced in neighboring metal.
•Deflecting torque is not exactly proportional to the square of the current due to non-
linear characteristics of iron material.
• Error due to residual magnetism in the
• The error can be minimized by reversing
the meter connections and averaging the
MI for DC purpose
• Non linear scale
• Draw more power
• Shielding (laminated iron cylinder) should be
provided to protect from external magnetic fields
• Deflecting torque is not exactly proportional to
• Variation of frequency introduce error
Types of Errors
•All the human mistakes while reading and recording.
•Mistakes in calculating the errors also come under this category.
Ex. while taking the reading from the meter of the instrument he
may read 21 as 31. All these types of error are come under this
Avoided by using suitable measures as:
•A proper care should be taken in reading, recording the data.
•Calculation of error should be done accurately.
•As increasing the number of experimenters we can reduce the
gross errors. If each experimenter takes different reading at
different points, then by taking average of more readings we can
reduce the gross errors.
2. Systematic Errors
2.1 Instrumental Errors
•Due to wrong construction, calibration of the measuring instruments
•Arises due to friction or may be due to hysteresis.
•Include the loading effect and misuse of the instruments which
results in the failure to the adjust the zero of instruments.
•To minimize this, various correction factors must be applied and in
extreme condition instrument must be re-calibrated carefully.
2.2 Environmental Errors
•External condition includes temperature, pressure, humidity or it
may include external magnetic field.
To minimize this error:
•Try to maintain the temperature and humidity of the laboratory
constant by making some arrangements.
•Ensure that there should not be any external magnetic or
2.3 Observational Errors
•Due to wrong observations.
•The wrong observations may be due to PARALLAX.
•To minimize the PARALLAX error highly accurate meters are
required, provided with mirrored scales.
2.4 Random Errors
After calculating all systematic errors, it is found that there are still
some errors in measurement are left. These errors are known as
Some of the reasons of the appearance of these errors are known but
still some reasons are unknown. Hence we cannot fully eliminate
these kinds of error.