Sensing AC magnetic fields
PARTS AND MATERIALS
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
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 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! |