Our story begins with the commonplace magnetic field, as shown here in this picture of a popular elementary school experiment. (This image has been released into the public domain and is from the book Practical Physics, published in 1914 by Macmillian & Company. I copied it from the Wikipedia.) In the experiment, a piece of paper is placed on top of a bar magnet and iron filings are sprinkled over the paper. When the paper is tapped, the filings line up along lines of magnetic flux. The magnetic field acts on moving electrical charges and exerts a force on the charges that is perpendicular to the field and to the direction of motion of the charges. Although the filings only line up along some of the field lines, field lines actually fill all of the space around the magnet, propagating out from the magnet at the speed of light and eventually extending out infinitely far. Any charged particle moving within the field will encounter a field line and experience a corresponding magnetic force. The force will be smaller the further the particle is away from the magnet.