EMF meters: measuring your own magnetic fields
The single most useful thing a person worried about EMF can do is stop guessing. A usable gaussmeter costs less than most people expect.
What you are measuring
Household power in North America alternates at 60 Hz (50 Hz in much of the world). The magnetic fields it produces are measured in milligauss (mG) - or, in SI units, microtesla (µT), where 1 µT = 10 mG. Typical readings in the middle of a room away from appliances run 0.2–2 mG. Epidemiological studies of residential exposure have generally drawn their comparison line in the 2–4 mG range, which is why those numbers recur in the literature; they are study thresholds, not established safety limits. Appliances produce locally intense fields (a hair dryer can read hundreds of mG at the grill) that collapse within a foot or two - it is the steady, room-filling fields from net currents and nearby lines that the measurement effort is really about.
Single-axis vs three-axis meters
A magnetic field has direction. A single-axis meter contains one sensing coil and reads only the component of the field aligned with it - rotate the meter and the reading swings from maximum to nearly zero. A three-axis meter contains three orthogonal sensors and combines them, so the reading is essentially independent of how you hold it.
| Three-axis | Single-axis | |
|---|---|---|
| Ease of surveying | Point-and-read; ideal for walking a house | Requires rotating at each spot to find the maximum |
| Tracing a source | Tells you how much | Tells you which way - the directionality that makes source-hunting possible |
| Typical price | Moderate | Often cheapest |
The practical answer is that the two types are partners, not rivals. Survey with a three-axis meter to find where the field is; switch to single-axis thinking (most three-axis meters have a single-axis mode) to find what direction it comes from. Edward Leeper worked out a simple convention for using a single-axis sensor to walk down the direction of a field source - a method still referenced in the instruction manuals of commercial gaussmeters - and his book treated the inexpensive single-axis meter as entirely adequate for solving real problems.
Choosing a meter
Features that matter, in rough order:
| Feature | Why it matters |
|---|---|
| Resolution to 0.1 mG | Room-level problems live in the 1–10 mG range; a meter that reads in whole milligauss hides the story. |
| Frequency response / weighting | You want the meter honest at 60 Hz and its harmonics (180, 300 Hz…). Some meters "frequency-weight" readings upward at higher frequencies - fine for some purposes, but know which behavior yours has. |
| ELF/VLF band separation | Separating power-frequency fields from higher-frequency sources (induction cooktops, some electronics) keeps diagnoses clean. |
| An analog needle or fast bar-graph | Watching a value rise and fall as you sweep is how tracing actually works; slow digital updates fight you. |
Long-standing instruments in this space include AlphaLab's gaussmeters (the maker whose manuals cite this site's namesake book) and the TriField family of meters; both have been fixtures among electricians, building inspectors, and audio engineers for decades. Radio-frequency "EMF meters" marketed for ghost hunting and 5G anxiety are a different product category - for power-frequency wiring problems you want an instrument specified in milligauss at 50/60 Hz, with published accuracy.
How to survey a house
A useful survey is systematic and boring, in the best way:
1. Map first. With everything in its normal state, walk each room at waist height and note readings at the center, at the bed pillow, and anywhere people sit for hours. Write numbers down; memory flattens gradients.
2. Chase the gradient. Where readings rise, keep moving in the direction of increase. Line sources (pipe runs, cable runs, service drops) reveal themselves as ridges of field, not points.
3. Experiment with breakers. Kill circuits one at a time and watch the meter. A reading that dies with a breaker implicates that circuit's wiring. A reading that survives the main breaker points outside your walls - the water service carrying neighborhood return current, or a nearby line (see the power-lines page).
4. Re-measure at different hours. Net-current fields follow electrical load. Readings often double in the evening when the neighborhood cooks dinner; a morning-only survey can miss the problem entirely.