Whenever electricity (i.e., batteries-DC or electric
power-AC) is in use, electric and magnetic fields are produced. If it
is alternating current (AC) electric power, the fields fluctuate (expand
and collapse) 120 times every second as the 60 cycles-per-second (known
as Hertz) alternating current (AC) changes polarity. Since the AC frequency
is 60 Hertz it emanates extremely low frequency (ELF) electromagnetic
fields (EMF) - hence the term ELF EMF.
Electric fields emanate from unshielded power lines, wires,
equipment, and appliances. The electric field strength is measured in
volts per meter (V/m). So under high voltage transmission lines (230
- 500 kV) there are very high electric fields -- people can usually hear
the crackle and feel the presence on their skin. Fortunately, electric
fields can be shielded by grounded objects and materials including metal
conduits, trees, and buildings.
Magnetic fields also emanate from unshielded power lines,
wires, equipment, and appliances. The magnetic field strength is measured
in amperes per meter (A/m) and is proportional to the load current. Therefore,
high current loads such as transmission and distribution lines, transformers,
feeders, switchgears, and heaters produce high magnetic field levels.
Unfortunately, people are not able to sense the presence of high magnetic
fields which are extremely difficult to shield -- permeating people,
buildings, and most metals.
Magnetic field exposure is measured as magnetic flux density
with a gaussmeter in units of milligauss (mG), which is one-thousandth
of a Gauss (G). In scientific terms Gauss (G) is the standard U.S. unit
of magnetic flux density -- the area permeated by magnetic fields. Therefore,
in the United States human exposure to magnetic fields is normally measured
and published in milligauss (mG). It should be noted that in Europe microtesla
(µT) is the preferred unit - where 1 mG equals 0.1 µT.
Finally, there are both single and triple-axis gaussmeters
on the market. A single axis gaussmeter has a magnetic probe (coil, fluxgate
or hall-effect) that is sensitive in only one direction: the meter reading
is dependent on the orientation of the probe. A triple-axis gaussmeter
has three separate magnetic probes placed on the x, y, and z axis. This
meter electronically computes the vector sum of the directional components
and displays the resultant on a digital readout.
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