The device that operates using the magnetic effect of electric current is

Analysis

The question asks us to identify which of the listed devices functions based on the magnetic effect of electric current (also known as electromagnetism).

Concept Explanation

When an electric current flows through a conductor, it creates a magnetic field around it. This principle is the foundation of electromagnetism. Devices that utilize this effect typically use an electromagnet—a coil of wire wrapped around a soft iron core that becomes magnetized only when current flows through it.

Let's evaluate the options:

• A. Rheostat: This is a variable resistor used to control the flow of current in a circuit. It operates based on the principle of resistance, not magnetism.
• B. Thermostat: This is a device used to maintain a constant temperature. It typically operates based on the principle of thermal expansion of metals (bimetallic strips).
• C. Electric bell: This device contains an electromagnet. When the switch is pressed, current flows through the coil, creating a magnetic field that attracts a soft iron armature, causing the hammer to strike the bell. Once the circuit is broken, the magnetism ceases, and the process repeats.
• D. Carbon microphone: This device converts sound waves into electrical signals based on changes in electrical resistance caused by pressure on carbon granules.

Step-by-Step Solution

1. Identify the core principle: We are looking for a device that converts electrical energy into mechanical movement using magnetism.
2. Examine the Electric Bell: An electric bell uses an electromagnet to pull a clapper against a gong. Without the magnetic effect of the current, the clapper would not move.
3. Eliminate others: The Rheostat (Resistance), Thermostat (Thermal expansion), and Carbon microphone (Variable resistance) do not rely on the magnetic effect of current for their primary operation.
4. Conclusion: The electric bell is the classic example of an application of the magnetic effect of electric current in physics.

Answer

The correct option is C.

Exam Tip

Whenever you see a question about the "magnetic effect of current," look for devices that involve movement or switching caused by electricity, such as electric bells, electric motors, relays, or circuit breakers. These are the most common examples tested in JAMB Physics.