Measuring One's Own Magnetic Fields


Much of what is covered in Silencing the Fields depends on taking AC magnetic field measurements. And it is inconvenient always to rely on someone else to do that for you.

As is explained in Silencing the Fields, various reasonably priced AC magnetic field meters (EMF meters) are available in two types: A "single axis" or directional meter measures the AC magnetic field strength in only one direction at a time. This strength-in-a-direction is called the field's "component" in that direction - usually either perpendicular to the face of the meter, or along the length of the meter. To determine the field's total strength (rather than just its strength in one direction) one usually tips the meter to various orientations, looking for an orientation that gives the greatest reading. This is not always explained very well in the meter's directions, and it can be tedious to do (did you really try all the possible orientations?) - especially if one is simultaneously trying to find the location that gives the highest reading (near a suspected field source, say).

Moreover, unless we develop some special tricks, the tedium with a single-axis meter becomes even greater if the meter is digital - because comparing one set of digits with another set we saw a second earlier (as we move or rotate the meter looking for a maximum) is inherently slower than watching whether a pointer goes up or down.

Thus mistakes tend to get made when using a single axis EMF meter. For instance, we may start by correctly determining the field's orientation at a particular place in a room (by rotating the meter to a maximum reading there) but then we may try to move the meter around the room to find if there is a higher-field location, without remembering to do further checks on the field's angle to make sure we're still pointing it correctly. Especially if a field's source is close by, the field angle may change in a short distance. We may actually move the single axis meter toward this source but see the readings go down, because we're no longer holding the meter at the maximum-field orientation.

All this can be a real pain. One solution is to spend roughly an extra hundred dollars (give or take) to buy a "three axis" meter - a non-directional kind that takes three simultaneous single-axis readings in three mutually perpendicular directions, and then combines them electronically to give a "resultant" reading that is usually the same field strength as we would get by rotating the meter to a maximum reading.

The only other good solution is to get the best, most-convenient single axis meter (i.e. one that responds quickly but steadily and legibly when rotated) and then learn a bag of tricks that speed things up. For instance, in many situations, vertical or near-vertical is the most likely field orientation. Thus a useful trick for using a single-axis meter is to start with the meter held so as to read a vertical field - and then tip it forward and back, and left and right, to see if our first guess is correct, or if another angle gives us more. That is really not a bad technique, using a good single axis meter.

The next important trick is to use a prior knowledge of the field angle we expect from a particular source - perhaps a power line that we see in front of us, or a current-carrying water line that we know is under our feet - and let that give us our "first guess" as to the maximum-reading field direction. Again, not a bad technique.

But this is more than just a way to get a quick reading. What this technique also does for us is to tell us if our assumption is correct about what is causing the fields we're seeing. If the fields point in some other direction, then there must be another source that we've missed - perhaps a different current-carrying pipe or set of wires, and not the one we were looking at. With a three axis meter, we don't get that kind of reality check; we just see nebulous areas of elevated fields. We may make mistakes, trying to work without the full information including the field's direction; and we may persist in a wrong diagnosis, and waste time that way.

In fact, it is quite a common mistake in planning field mitigation that something else is causing the fields besides what seems obvious at first. We need help from every clue we can get, including field direction. Deliberately throwing away that information makes things harder rather than easier. Of course we have to know how to use the directional information once we get it, but it's not that hard to learn.

Hence Silencing the Fields is written with the assumption that a good single axis meter will be needed for identifying field sources (for instance, for tracing water lines under a yard or street, or for determining if the fields from a power line are due to a net current or due to the separation of the wires - with different fixes needed - and so on).

If you already have a three-axis meter, or want to get one, it will indeed be handy for many purposes. But for serious source diagnosis and mitigation, a good single axis meter is needed. There are several available between $100 and $200 that are responsive, but not too jittery, though most EMF meters in that price range are digital, which slows things down. You may want to own two meters.

But there is also a trick described in Silencing the Fields for temporarily modifying a three axis meter to read as a single-axis meter - by holding a piece of magnetic metal next to it so as to "shunt the field" past two of the meter's three single-axis detectors, leaving only one axis fully active.

The Silencing the Fields section on magnetic field meters (Part IX) has 68 pages, 26 photographs, and covers the strengths and weaknesses of 35 different makes and models. There is no ideal meter (inexpensive but easy to use and fairly accurate); but a number, in different price ranges, come close.

Silencing the Fields - Practical guidance for EMF meter selection

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There are two standard types of single-axis meter (two detector-coil orientations used inside the meter). In these line drawings from page 33 of Silencing the Fields, their best "tipping procedure" for finding the maximum-field strength and direction are shown. These procedures simplify and speed up the use of a single-axis meter, but as of 11/15/01 only one or two single-axis meters were sold with such instructions included.

Several other meters employ a third coil orientation (crosswise) that is more awkward to use for many purposes. Silencing the Fields identifies these meters and suggests how to use them If they are already purchased.

Silencing the Fields lists the coil orientations that are used in most of the available single-axis meters. That information is supplied with only a few meters, or listed in their advertisements. Other meter makers leave you to deduce or guess at what the "active axis" is. (One even has an active axis that is tilted thirty degrees.)

EMF Meters