Measuring device
For the measurement of magnetic fields a measuring device incorporating one or several copper-wire coils is normally used. Magnetic fields from electricity supply installations change direction backwards and forwards 50 times a second. When a magnetic field moves relative to a copper-wire coil, an electric current is produced in the coil windings. In principle, it is the same that happens in a generator.

The amperage created in the coil windings depends on the strength of the magnetic field. The electronics in the measuring device translates the electric current created in the coil to a measurement of the magnetic field strength. Magnetic fields from electricity supply installations are normally measured in microteslas (µT).

Different sources of fields
Close to, for example, high-voltage power lines the greater part of the field measured originates from the power line. Close to household appliances the greater part of the field may come from the appliance. Field strengths drop rapidly with distance, however. It is seldom possible to distinguish fields produced by the appliance from the ordinary background field in the home up to one metre from the most common appliances.

Whether or not there are high-voltage power lines in the vicinity of a home, there will always be a certain background field in the home that is produced by the ordinary house installations - provided all appliances have not been disconnected. The strength of the background field may differ a great deal from home to home, but it is usually quite small.