Electromagnetic field
Natural electromagnetic fields are present everywhere in the
environment but are invisible to the human eye. Electric fields are produced by
the build up of electric charges in the atmosphere. The Earth's magnetic field
causes a compass needle to point in a north-south direction and is used by
birds and fish for navigation.
Man-made electromagnetic fields include X-rays, and the
electricity that comes out of every power socket has associated low frequency
electromagnetic fields. Various kinds of higher frequency radio waves are used
to transmit information, for instance TV antennas, radio stations or mobile
phone base stations.