# 4.06: Sensing AC Magnetic Fields

PARTS AND MATERIALS

• Electromagnet coil from relay or solenoid

What is needed for an electromagnet coil is a coil with many turns of wire, so as to produce the most voltage possible from induction with stray magnetic fields? The coil taken from an old relay or solenoid works well for this purpose.

CROSS-REFERENCES

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

LEARNING OBJECTIVES

• Effects of electromagnetic induction.
• Electromagnetic shielding techniques.

SCHEMATIC DIAGRAM

ILLUSTRATION

INSTRUCTIONS

Using the audio detector circuit explained earlier to detect AC voltage in the audio frequencies, a coil of wire may serve as a sensor of AC magnetic fields. The voltages produced by the coil will be quite small, so it is advisable to adjust the detector’s sensitivity control to “maximum.”

There are many sources of AC magnetic fields to be found in the average home. Try, for instance, holding the coil close to a television screen or circuit-breaker box. The coil’s orientation is every bit as important as its proximity to the source, as you will soon discover on your own! If you want to listen to more interesting tones, try holding the coil close to the motherboard of an operating computer (be careful not to “short” any connections together on the computer’s circuit board with any exposed metal parts on the sensing coil!), or to its hard drive while a read/write operation is taking place.

One very strong source of AC magnetic fields is the home-made transformer project described earlier. Try experimenting with various degrees of “coupling” between the coils (the steel bars tightly fastened together, versus loosely fastened, versus dismantled). Another source is the variable inductor and lamp circuit described in another section of this chapter.

Note that physical contact with a magnetic field source is unnecessary: magnetic fields extend through space quite easily. You may also want to try “shielding” the coil from a strong source using various materials. Try aluminum foil, paper, sheet steel, plastic, or whatever other materials you can think of. What materials work best? Why? What angles (orientations) of coil position minimize magnetic coupling (result in a minimum of detected signal)? What does this tell us regarding inductor positioning if inter-circuit interference from other inductors is a bad thing?

Whether or not stray magnetic fields like these pose any health hazard to the human body is a hotly debated subject. One thing is clear: in today’s modern society, low-level magnetic fields of all frequencies are easy to find!