Detecting AC Magnetic Fields with a Coil and Audio Detector

PARTS AND MATERIALS

• Audio detector with headphones
• Electromagnet coil from relay or solenoid

A high‑turn count coil is essential to maximize induced voltage from stray magnetic fields. Coils salvaged from old relays or solenoids provide an ideal, readily available option.

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 2, chapter 7: “Mixed‑Frequency AC Signals”

LEARNING OBJECTIVES

• To determine the effects of electromagnetic induction.
• To determine the electromagnetic shielding techniques.

SCHEMATIC DIAGRAM

ILLUSTRATION

INSTRUCTIONS

By pairing the audio detector circuit with a multi‑turn coil, you can listen for AC magnetic field‑induced voltages. Because the induced signals are typically weak, set the detector’s sensitivity to its maximum setting.

Common household sources of AC magnetic fields include televisions, power panels, and even the inductors in household appliances. Bring the coil close to these devices to hear the resulting tones.

The coil’s orientation relative to the field lines dramatically influences the detected signal. Experiment by positioning the coil against a computer motherboard—taking care to avoid shorting any circuitry—or against a hard drive during active read/write operations for richer tones.

A homemade transformer is a potent source of AC magnetic fields. Test different coupling levels by varying the proximity of the primary and secondary coils—tight, loose, or completely separated—to observe how coupling affects the detected signal.

The variable inductor and lamp circuit, detailed elsewhere in this chapter, also emits measurable AC magnetic fields.

Physical contact isn’t required; magnetic fields permeate space. You can also experiment with shielding by surrounding the coil with materials such as aluminum foil, paper, sheet steel, or plastic to study attenuation effects.

Test different shielding materials—aluminum foil, paper, sheet steel, plastic—and record which offers the greatest attenuation. Consider how coil orientation and shielding angle influence coupling. These observations guide optimal inductor placement to mitigate inter‑circuit interference.

The health implications of low‑level magnetic fields remain debated, but modern life is replete with such fields across a wide frequency spectrum.

RELATED WORKSHEET:

• Basic Electromagnetism and Electromagnetic Induction Worksheet
• Intermediate Electromagnetism and Electromagnetic Induction Worksheet