Lesson Notes By Weeks and Term v4 - SHS 2

Lesson Video

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Performance objectives

• Explain the principle of operation of moving iron instruments.
• Describe the construction of these instruments.
• Identify their advantages and disadvantages.
• Convert a moving iron instrument into a voltmeter and an ammeter.

Lesson summary

In our daily lives in Ghana, from the ECG technician reading our home meter to the phone repairer at Circle checking a circuit, measuring electricity is essential. The tools they use, like ammeters and voltmeters, are crucial. But what if you have a basic measuring device that can only handle a tiny current? Do you throw it away? No! In engineering, we are resourceful. We learn to adapt and modify. This lesson is about a powerful technique: how to take a basic, sensitive measuring instrument—the Moving Iron (MI) instrument—and cleverly convert it to measure much larger currents or voltages, making it a versatile tool for any technical job.

Lesson notes

2.1 The Moving Iron (MI) Instrument

A Moving Iron instrument is a type of measuring device used to measure AC and DC currents and voltages. It's known for being robust and relatively inexpensive.

A. Principle of Operation (Repulsion Type) The operation is based on a simple magnetic principle: like poles repel. Electromagnetism: The current to be measured is passed through a fixed coil of wire. This turns the coil into an electromagnet. The strength of this electromagnet is proportional to the current flowing through it. Magnetisation: Inside the coil, there are two small pieces of soft iron. One is fixed to the coil's frame, and the other is attached to a spindle, which is free to rotate. Repulsion: When current flows, the magnetic field from the coil magnetises both pieces of iron. Crucially, they are magnetised with the *same polarity* facing each other (e.g., both become North poles on the same side). Movement: Because like poles repel, the two pieces of iron push away from each other. Since one is fixed, the movable iron vane is forced to move. Indication: This movement causes the spindle to rotate, which in turn moves a pointer across a calibrated scale, giving a reading. The greater the current, the stronger the magnetic field, the greater the repulsion, and the further the pointer moves.

Key takeaway: The deflection (how much the pointer moves) is proportional to the square of the current (I²). This is why it works for both AC and DC. For AC, the current reverses, but the repulsion is always there ((-I)² = I²). It also means the scale is not linear; it's cramped at the beginning and spread out at the end.

Evaluation guide

• Convert moving iron instrument into voltmeter and ammeter
• Collaborative Learning: With the use of the internet, engage learners in mixed ability
• groups to undergo the following activities:
• Explain principle of operation of the moving iron measuring instruments