Measuring Electric and Magnetic Fields
Using a common lamp as an example, electric fields are present
(measured in volts per meter) when the lamp is plugged in. When the lamp
is plugged in and turned on, the electrical charges move, and magnetic
fields are created (measured in milliGauss, mG). Because electric fields
and magnetic fields have different characteristics, different measuring
instruments are necessary in order to assess emission levels for each type
Measuring Magnetic Fields
It is impossible to avoid exposure to low frequency magnetic fields
in modern society - some level of background radiation will always be
present. This background radiation can arise from a number of
sources and must be taken into consideration when
measuring electromagnetic fields from a particular appliance or source. Before any
assessment of the emissions from any source,
it is necessary to first determine the strength of the
background field. In some cases, the background field might be greater than
that from the source, making measurements of the
emission from the source impossible. In order to determine background radiation levels,
the device to be measured is turned off and a set of readings taken
in the area around it. If the background radiation is high, (for example greater than
5 mG), the contribution of the device to the environment may not
be measurable. (In recognition of this fact, the Swedish specification
MPR II requires that the background levels be no greater than 0.4
milliGauss in order for testing to be valid.) If two sources are close
enough for their fields to interact, they could amplify each other or
cancel each other, depending on the distance and field direction.
This is called constructive and destructive interference. Thus,
it is possible for the field strength measurement to increase after
turning off a source.
Fields and Measurement Devices are Directional
Differences among magnetic field meters can be considerable, so that
it is important to know a meter's capabilities and limitations before using it for
test and measurement purposes. A well-designed radiation meter will not only
tell the strength of the field, it
should also be directional. The meter should measure fields in one
direction or plane at a time and display the maximum field
strength at that location and at that angle. However, a person at that
location may be exposed to fields coming from all several directions
simultaneously. In order to determine the maximum field strength at a
particular location, the radiation meter should be rotated through all possible directions so
that the field which is present can intersect with the sensor in such
a way as to produce a maximum reading. In order to determine exposure at
a particular location, the maximum field strength measurement in three
planes (x, y and z) are measured and the square root of the sum of the
squares of the individual readings is determined. The resulting RMS value
(root-mean-square) is the measured value of electromagnetic radiation at that location.
The common unit of measrement of magnetic field is the Gauss or milligauss.
Measuring Electric Fields
Electric fields are produced by differences in electric potential or voltage
For example, if a 9 volt battery is
attached to two metal plates
one meter apart, an electric field would
exist between them given by the voltage divided by the distance,
or 9V/m. If the voltage difference remains constant, the field is
said to be static and is called a static electric field.
Static electric fields are quite common
and can be relatively high. Static fields in excess of several
thousand volts per meter often exixt in front of
a computer monitor. Such static fields can cause skin irritation and
dryness of the eyes, especially for contact lens wearers.
If the voltage between objects changes with
time, the field is called a alternating electric field,
or an alternating current (AC)
field. The rate at which the field changes is called its
frequency, expressed in Hertz (Hz).
The objective of a
measuring instrument is to determine the average field strength
at the specified position. Although several methods are available,
the usual method is the RMS (root-mean-square) average.
Measurements are taken of the maximum field strength reading in three perpendicular planes and
the RMS average is then equal to the the square root of the sum of the squares of the
individual readings. If the field is alternating or oscillating
at a fixed frequency such as 60 Hz in is the case of power lines,
an electric field meter can be set such that it has maximum
sensitivity at these power frequencies. However, if
the electric fields consist of a number of frequencies,
a limitation must be set.
The range of frequencies which are
allowed to be represented in the RMS average is called the
bandwidth of the instrument and is expressed as a range of
frequencies (for example 10 - 1000 Hz). When choosing an
electric field meter it is important that the instrument
is designed to read the specific frequencies to be measured.
The common unit of electric field measurement is V/m.