You can start by turning off the main breaker on your house. That kills all the power in the wiring. If the magnetic field in the room drops to near zero, you know the source is inside your house, not a power line outside. To check this you need a gaussmeter, which is a small handheld device that reads magnetic fields in milligauss. You can buy one for a reasonable price from an electronics store or rent one from a local library that lends tools.

Walk around the room with the meter held away from your body. Hold it at arm’s length and keep it still for a few seconds in each spot. The meter will give you a number. Most rooms in a typical house read under 1 milligauss when you are standing away from appliances and walls. If you see a reading of several milligauss or more across a whole room, the wiring is probably the cause.

A common problem is a circuit where the outgoing current and the returning current do not cancel each other out. That happens when the two wires are not run close together in the same cable. Sometimes the current finds its way back through a water pipe or the ground wire. That can create a loop that fills a room with a steady magnetic field. A licensed electrician can trace these currents with a clamp meter. They can fix the problem by reconnecting the wires properly or by installing an isolation device on a water pipe.

Another surprise comes from an open neutral or a bad connection in the panel. That can push current onto paths it was never meant to take. The electrician will check each circuit and look for what is called a net current on the ground wire. If the reading on the ground wire is above a very small fraction of the line current, there is a wiring fault that needs fixing.

On health, the International Agency for Research on Cancer classified extremely low frequency magnetic fields as possibly carcinogenic in 2002. That classification was based on studies that found a link between higher average exposure and childhood leukemia. Very few children live in houses with the levels that those studies looked at, and the studies cannot prove that the magnetic field caused the illness. For comparison, the international reference level for the public is 2,000 milligauss, far above anything you will measure in a home.

If you have a health concern, talk to your doctor. If you want to know what the utility company is responsible for, call them. They own the lines up to your meter. Inside the house, it is your wiring. A licensed electrician is the right person for that job. Do not use stickers, shields or devices that claim to block or reduce magnetic fields. Those products have no scientific backing and can give you a false sense of security.

A closer look is at Field at the Fence.