When electricity flows through a wire, we can compare it to water flowing through a hosepipe; the current would be the volume of water, and the voltage would be the pressure pushing the water through the hosepipe. To transfer electricity, we always have a voltage and a current. 

Magnetic fields are produced by the current flowing through a wire. The strength of the magnetic field from a substation, overhead line or cable can go up and down, because the current goes up and down depending on the electricity demand in that area. Generally, the higher the current, the higher the magnetic field.

Magnetic fields are not easily screened; they pass through most things. Because of this, cables buried underground will still produce a magnetic field, which you can measure on top of them. 

Magnetic fields are usually measured in microtesla (µT) or nanotesla (nT). One nanotesla is one thousandth of a microtesla (1 nanotesla = 0.001 microtesla). Sometimes, milligauss (G) is used, where 1 milligauss is one tenth of a microtesla (1 milligauss = 0.1 microtesla = 100 nanotesla).  Current, which is the main factor in determining the magnetic field, is measured in Amperes (Amps). 

Electrical appliances and wiring that is normally used in houses generate a magnetic field, so background magnetic fields are present in most homes. The magnetic field level in a house in the UK will typically be between 0.01 to 0.2 microtesla (µT).

Magnetic fields are vectors; that means they have both a strength (that we can measure or calculate) and a direction. That’s important when we look at overhead lines, because the magnetic fields produced by each set of wires can interact with one another and alter the magnetic field we measure. The way many overhead lines are designed means that the magnetic fields on each side of a pylon can cancel each other to a degree. More information on this can be found under the Phasing, transposed phasing, untransposed phasing, and optimum phasing heading here. 

The magnetic fields produced by each set of wires on an overhead line combine depending on how they are arranged. This picture gives an example of the magnetic field lines emanating from the wires.

The Earth has a magnetic field, produced by currents deep inside the molten core of the planet. The Earth's natural magnetic field is approximately 50 µT in the UK. However, these are different to those produced by the electricity system and are discussed further here.