An electromagnetic field, sometimes referred to as an EM field, is generated whenever charged particles, such as electrons, are accelerated. All electrically charged particles are surrounded by electric fields. Charged particles in motion produce magnetic fields. When the velocity of a charged particle changes ( acceleration or deceleration ), an EM field is produced.

Electromagnetic fields were first discovered in 1867, when professor James Clerk Maxwell, or the university of Edinburgh, suggested the radically new concept of electricity and magnetism.  He outlined theoretically the exact type of electromagnetic wave the is used in radio to day, and predicted perfectly its behavior.  It was not until 1886 that Heinrich Hertz, working in Karlsruhe, Germany, confirmed Maxwell's theory by creating and detecting these electromagnetic waves.  Hertz electromagnetic generator consisted of a spark gap to which was attached a pair of outwardly extending conductors with flat plates on the ends ( a form of center feed antenna ).  Hertz  noticed that electric arcs (sparks) could be reproduced at a distance, with no connecting wires in between. Hi's receiver was a wire ring with a small gap at one point.  When electromagnetic waves from his generator arrived at the receiver ring a small spark could be observed across the gap.  Hertz experiments led leading scientists in several countries to believe that it was possible to communicate over long distances without wires.

A young student of one of theses scientists, Guglielmo Marconi, recognized the defects in the devices previously used to test for the arrival of electrical waves.   To him came not only the idea that these invisible electromagnetic waves could be used for wireless telegraphic signaling, but also the inspiration that led to practical solutions of the many problems involved in producing a set of sending and receiving instruments.  His first radio transmitters made use of electric arcs to generate electromagnetic radiation just as Hertz had earlier.  These "spark transmitters" and the associated receivers were as exciting to people in the early 20th century as the Internet is today. In September 1896 Marconi sent a telegraph message by radio a distance of one and 3/4 miles in Salisbury, England. The next year ( September, 1897 ) he returned to the Salisbury plain, this time using the balloons to support the wire antennas up to approximately 150ft. With this arangment he obtained good signals with his receiver at Pitton, a distance of three miles, at Farley (3.8 miles) and between East and West Grimstead (5 miles). Weaker signals were received at Pepperbox (6.7 miles) and at Winterbourne Road (3.8 miles).

Electromagnetic fields are typically generated by alternating current (AC) in electrical conductors. The frequency of the AC can range from hundreds of cycles per second (at the low extreme) to trillions or quadrillions of cycles per second (at the high extreme). The standard unit of EM frequency is the Hertz, abbreviated Hz. Larger units are often used. A frequency of 1,000 Hz is one kilohertz (kHz); a frequency of 1,000 kHz is one megahertz (MHz); a frequency of 1,000 MHz is one gigahertz (GHz).

The wavelength of an EM field is related to the frequency. If the frequency F of an EM wave is specified in megahertz and the wavelength w is specified in meters (m), then in free space, the two are related according to the formula

w = 300 / F

For example, a signal at 100 MHz (in the middle of the American FM broadcast band) has a wavelength of 3 m, or about 10 feet. Wireless assistive systems operating at 74 MHz. ( the middle of the band ) have a wavelength of approximately 4 m., while at 216 MHz. the wavelength is only 1.4 m or about 55 inches.

Electromagnets fields are used for many important applications decide wireless transmission.  Scientists have known since the early part of the 19th century about the connection between electrical fields and magnetic fields.  Moving electric charge (electric current) creates a magnetic field. Coils of wire can be used to add the small fields made by each loop to create very large electromagnets such as those used in car junk yards or the much smaller electromagnets in your telephone receiver or stereo speakers. Electric motors used to start your car or spin a computer's hard disk around are other applications of this phenomenon. In fact, ordinary magnets are produced from tiny currents at the atomic level.

A conductor in a changing magnetic field or a conductor moving through a fixed field creates electrical an current. This concept is used by power generators---large coils of wire in a magnetic field, made to turn  (by falling water, wind, or by steam from the heating of water by burning coal or oil or the heat from nuclear reactions). The coils of wire as they rotate cut throung the fixed magnetic field and electricity is produced.

Computer disks and audio and video tapes encode information in magnetic patterns on tapes of disk covered in iron particles. When the blank ( un-magnetized ) media passes below a coil some of the iron particles become positively or negatively magnetized in proportion to the instantaneous signal level. When the magnetic disk or tape material again passes by the small coils of wire in the playback device, electrical currents ( signals ) are induced in the coils reproducing the original content.

Visable light is actually a type of electromagnetic radiation, occupying only a small portion of the possible spectrum of this electromagnetic energy. The various types of electromagnetic radiation differ only in wavelength and frequency; they are alike in all other respects