The magnetic relay is a device used for

Analyze the question: It asks what a magnetic relay is used for.

Explain the concept: A relay is an electromagnetic switch. A small current in a coil (the control circuit) magnetizes an iron core. The core attracts an iron armature, which closes (or opens) a separate set of contacts. Those contacts switch a second circuit, which can carry a much larger current or voltage.

Step-by-step solution:

1. Low-current circuit: A small current flows through the relay's electromagnet coil.
2. Magnetization: The coil magnetizes the soft iron core.
3. Armature attraction: The core pulls the soft iron armature toward it.
4. Contact closure: The armature movement closes the contacts of a second circuit.
5. Result: The larger-current circuit (such as a motor, a car starter or a heavy lighting circuit) is switched on by a small, safe current.

Checking the options:

• A. Timing in sports: this is done by a stopwatch or a timer, not a relay. ✗
• B. Storing a magnetic field: this is not what a relay does. ✗
• C. Reading magnetic flux: this is done by a fluxmeter or a magnetometer. ✗
• D. Controlling another circuit carrying a larger current: this is correct. ✓

Correct answer: D

Exam tip: Remember "Relay = Remote switch." A small current controls a big one. You will see relays in car starter circuits, in electric bells and in telephone systems.