Moving iron instruments are the most common type of ammeter and voltmeter used for laboratory or switchboard applications involving power frequencies. They can measure current and voltage with accuracy needed for most engineering works, and are cheaper than other types of AC instruments providing the same level of accuracy. Moving iron instruments are classified as either attraction type or repulsion type based on how the moving iron component moves within the instrument's coil in response to the magnetic field produced by the measured current or voltage.

1. Measurement and Instrument

2. Moving iron (M.I) Instrument
• The most common ammeter and voltmeter for
laboratory or switch board use at power
frequency are the moving iron instruments.
• The instrument can be constructed to
measure current and voltage to an accuracy
needed in most engineering works and still be
cheaper as compared any other type of a.c.
Instrument of same accuracy

3. Classification of moving iron
instrument
• Moving iron instrument are of two types
Attraction type
Repulsion type

4. Attraction type
• The construction of an attraction type moving
iron instrument. The coil is flat and has a
narrow slot like opening
• The moving iron disc or a sector essentially
mounted. When the current flows through the
coil a magnetic field is produced and the
moving iron moves from the weaker field
outside the coil to the stronger field inside it
or in other words the moving iron is attracted

5. • The controlling torque is provided by spring
but gravity control can be used for panel type
of instrument which are vertically mounted
• Damping is provided by air friction with help
of light aluminum piston

6. Repulsion type
• In the repulsion type there are two vanes
inside the coil one fixed other is movable
Radial vane type:
In this type the vanes are radial strips
of iron. The strips are placed within the coil
The fixed vanes is attached to the coil
and the movable one to the spindle of the
instrument.

7. Co-axial vanes type
in this type instrument the fixed and
moving vanes are sections of co-axial cylinder
as shown

8. • The controlling torque is provided by spring.
• The damping torque is provide by air friction.
• The operating magnetic field in moving iron
instrument is very weak and therefore eddy
current damping is not used in them as
introduction of a permanent magnet required
for eddy current damping would distort the
operating magnetic field.