|The constituents of atoms comprise small massive
entities held together by forces much stronger than that due to their
gravitational attraction. It is however not difficult to knock a few of these
constituents apart (e.g. by simply combing ones hair with a plastic comb on a
dry day) and to observe that they can both attract and repel one
This interaction is called the force due to electricity and the constituents are said to possess electric charge. The precise nature of the force between charged particles depends on their state of relative motion. Charges in motion are called electric currents and the forces between moving charges are referred to as electromagnetic. The simple bar magnet owes many of its properties to the motion of charges in its interior. The fact that electric charges can influence other charges across space is the fundamental reason that electrical forces can be harnessed as a source of usable power. It was Michael Faraday who conceived of the idea that such forces might be mediated by an entity in the space between the charges and that it could exist in its own right. This idea gave rise to the concept of the electromagnetic field. Just as the forces of sound are transmitted via the medium of air so the forces of electromagnetism are transmitted via the electromagnetic field. However unlike air the energy and momentum of the field is not associated with material particles like the molecules of the air. James Clarke Maxwell elucidated the structure of the field a few years later. He realised that charges in motion might generate oscillations in the field rather like a pebble in motion can set waves rippling across the surface of a pond.
It was not long before Heinrich Hertz was able to demonstrate this phenomenon in the laboratory. He was able to produce a spark in one corner of his laboratory by exciting the EM field into oscillation by means of a spark in another corner. According to Maxwell the influence was propagated across the room in a finite time. It soon became apparent that light itself was a periodic oscillation of this new field. These disturbances in the EM field travelled at enormous speed compared to the locomotion of everyday experience.
Electromagnetic oscillations generated on the West coast of England in 1901 by Marconi took only 0.01 seconds to cross the Atlantic Ocean where they excited the charges in a piece of metal in Newfoundland into motion. The refinement of this experiment led to the development of worldwide radio transmission and opened up a new window of exploration of he Universe.