What an EMF Meter Actually Measures

An EMF meter is one of the very few pieces of equipment in this hobby that is a genuine scientific instrument. Somebody designed it to answer a real question and it answers that question honestly. The difficulty is that almost nobody using one in a dark corridor knows what the question is.

What is in the box

Nearly every meter sold for paranormal work — the K-II and its many clones, the single-axis budget meters, the ones with the coloured LED ladder — contains a coil of wire. A changing magnetic field induces a voltage in a coil. The meter amplifies that voltage, filters it to a band around mains frequency, rectifies it, and lights the LEDs. That is the whole device.

Three consequences follow immediately and all three matter.

It measures change, not presence. A coil is blind to a field that is not moving. Hold a K-II next to a fridge magnet — a genuinely powerful static field — and nothing happens. Hold it next to a plugged-in phone charger drawing almost no current and it lights up. The instrument is not being perverse; it is doing exactly what a coil does.

It measures one axis, usually. A single-coil meter reads the component of the field along the coil. Turn it ninety degrees and the same field reads near zero. This is why two people standing in the same spot get different readings and conclude that something is responding to them. Three-axis meters exist, cost more, and are the correct tool.

It is tuned to your mains. The filter that makes the meter useful — rejecting radio, rejecting the earth's static field, rejecting your own movement — is the same filter that makes it a very good detector of domestic wiring and almost nothing else.

The K-II problem

The K-II is the meter everybody has seen, because it has five bright LEDs and no numeric display, and because a television programme made it famous. It is not a bad instrument. It is a single-axis, unfiltered-enough, non-latching meter with a coarse scale, sold for around a tenth of what a laboratory instrument costs, and within those limits it works.

What it is not is a communication device. The practice of asking a question and treating a flicker as an answer requires the meter to have been quiet before the question and to have responded because of it, and neither is usually established. A K-II placed on a floor in an ordinary house will flicker on its own several times an hour, because somebody upstairs switched a light, a fridge started, a phone in somebody's pocket negotiated with a mast, or the person holding it moved their arm through a gradient.

What actually moves the needle

The Survey's standing list, in rough order of how often it is the answer:

• Wiring in walls and floors, especially where live and neutral are not run together — a lighting circuit with a switch drop, an old two-plate system, a spur added badly.
• The consumer unit, and anything within two metres of it in any direction, including through a wall.
• Motors: fridge and freezer compressors, extractor fans, boiler pumps, anything with a duty cycle that will turn itself on halfway through your session.
• Transformers, which now means every plug-top charger in the building.
• Loudspeakers, which contain large permanent magnets, and which therefore affect a compass strongly and a mains-band coil meter hardly at all — a distinction worth holding on to.
• Underfloor heating, induction hobs, immersion heaters: large currents, low frequency, very obvious.
• Your own equipment. A camera, a torch, a phone and a battery pack all produce fields. Two investigators standing close together will read each other.

The first thing to do in a building

Find the consumer unit. Then find the meter cupboard, the boiler, the fridge and the loft hatch. Walk the whole building with the meter before anybody starts an investigation and mark on a plan every position that reads high and why. That plan is worth more than everything else you will do that night, because from then on a high reading is either on your map or it is not, and only one of those is interesting.

Where this site's detector fits

The detector on this station is not an EMF meter and does not claim to be. A browser is not allowed to read your phone's magnetometer directly, so no web page can report field strength in any unit at all.

What it reads instead is the compass heading, which is a magnetometer reading that has already been interpreted, and which carries the interference with it. When the field around the phone is disturbed, the heading stops being able to settle, and the scatter in it is a genuine physical measurement of that disturbance. It responds strongly to speakers and magnets — which a coil meter ignores — and much less to a distant mains cable, which a coil meter finds easily. Different instrument, different question, honestly labelled. The full account is in the instrument methodology.

Buying one

If you want a meter, buy a three-axis one and read its specification before you read its marketing. If the specification does not state a frequency band, the manufacturer either does not know or would rather you did not. Expect to pay meaningfully more than the LED-ladder instruments cost and to get a number rather than a mood.

And buy a notebook at the same time, because a reading you have not written down next to a time, a position and a compass bearing is not a measurement. It is a memory of a light.