Moving iron instruments are one of the earliest types of analog instruments used for the measurement of electrical parameters such as current, voltage, etc. They work on the principle of force of attraction between a stationary electromagnet coil and a moving iron vane or core placed near it. When current flows through the coil, it produces a magnetic field whose strength depends on the magnitude of current. This magnetic field then attracts the moving iron element placed inside the coil.

The moving iron, usually made of soft iron, experiences a force that causes it to move towards the coil. The amount of displacement is directly proportional to the strength of the magnetic field produced. By attaching a pointer to the moving iron element, its displacement can be visually indicated on a scale calibrated in terms of units of measurement like amperes, volts, etc. Thus, these instruments can measure the electrical quantity that produces the magnetic field i.e. current or voltage.

What is a Moving Iron Instrument?

Moving Iron (MI) instruments are popular electrical measuring devices used extensively for measuring alternating (AC) current and direct current (DC) flows. The scale of moving iron instrument is a nonuniform scale. Named after their operational component, these instruments harbor an iron piece that moves in response to the magnetic field generated by current flowing through a coil. Unlike devices that rely on permanent magnets, 

MI instruments use the electromagnetic interaction between current-carrying conductors and the moving iron element to translate electrical values into mechanical movements, thereby providing a visual representation of various electrical parameters such as current, voltage, and sometimes power.

The construction of Moving Iron instruments is fairly simple yet ingenious. The core component is a movable iron piece positioned inside or near a coil. When current flows through this coil, it creates a magnetic field that interacts with the iron piece, causing it to move. This movement is transferred to a pointer across a calibrated scale through a linkage mechanism. The instrument's body is usually made of non-magnetic materials to prevent interference with the magnetic field. A damping system is also incorporated to stabilize the pointer quickly without oscillations.

Fig-Moving iron instrument diagram

Working Principle of Moving Iron Instrument

The working principle of Moving Iron instruments revolves around the magnetic effects of electric current. When an electric current passes through the coil of the instrument, it generates a magnetic field. This field creates a force on a movable soft iron placed in the vicinity of the coil. 

The magnitude of the force, and thus the movement of the iron, is proportional to the square of the current flowing through the coil because the force is dependent on the magnetic field strength which itself is dependent on the current. This movement is mechanically linked to a pointer which moves over a scale to indicate the magnitude of the measured current or voltage. Since the force depends on the square of the current, MI instruments can measure both AC and DC with equal ease.

Torque Equation for MI Instruments

Or Torque equation of moving iron instrument:

Classification of The Moving Iron Instruments

Classification of the Moving Iron Instruments Moving Iron Instruments are classified mainly into two types based on their construction and operational characteristics: 

• Attraction type:  Attraction type moving iron instruments have a single piece of movable iron that is drawn into the coil as the current increases, indicating a higher measurement on scale.

• Repulsion type: Repulsion type instruments utilize two iron pieces within the magnetic field, which repel each other when current flows, moving the pointer. 

This classification allows for specific designs suited to varying measurement requirements and conditions. Both types are versatile and used for measuring current, voltage, and sometimes even power in electrical circuits.

Advantages And Disadvantages of Moving Iron Instruments

Here we will Discuss Advantages And Disadvantages of Moving Iron Instruments:

Advantages of The MI Instruments

Advantages of the MI Instruments Moving Iron Instruments come with several advantages. 

• They are inherently versatile and capable of measuring both AC and DC without any modifications. 

This universality makes them suitable for various applications. Their simple construction contributes to robustness, durability, and cost-effectiveness, making them a popular choice for basic electrical measurements. Additionally, they do not require external power sources for operation and can be designed to have fairly accurate linear scales, making readings easy and straightforward for users. A moving iron instrument can be used for measurement of Electric Voltage and Current.

Disadvantages of The MI Instruments

Disadvantages of the MI Instruments Despite their advantages, Moving Iron Instruments have several disadvantages. They are generally less accurate than other types of measuring instruments, particularly at the lower end of the scale due to non-linear scale spacing. Their accuracy is also affected by temperature changes and frequency variations in AC measurements. 

The presence of moving parts subjects them to mechanical wear over time, potentially affecting long-term reliability. Furthermore, these instruments are susceptible to external magnetic fields, which can lead to measurement errors if not properly shielded or compensated for.

Conclusion

Moving iron instruments were widely used in earlier decades due to their simple and robust construction. Though less accurate than modern electronic instruments, they continue to be used in certain applications due to affordability and reliability even today. The working principle of force of attraction between an electromagnet and movable iron forms the basis of various analytical instruments. Advances in materials and manufacturing have enabled development of improved variants of these early measurement devices.

This article summarises all the information related to Moving iron instruments, which helps in propelling your preparation for various AE/JE and ESE examinations. You can visit the Testbook app to keep yourself updated with all the exam-oriented information related to the upcoming examination, like SSC JE, GATE, ESE, RRB JE, and state AE/JE Exams.