Sorry to jump in, but when it comes to electromagnetism, see Minkowski's Space and Time on wiki. See this little paragraph towards the end, alongside figure 3: "In the description of the field caused by the electron itself, then it will appear that the division of the field into electric and magnetic forces is a relative one with respect to the time-axis assumed; the two forces considered together can most vividly be described by a certain analogy to the force-screw in mechanics; the analogy is, however, imperfect." Note that it's the field caused by the electron, and it's the two forces, one linear, the other rotational. The field is the electromagnetic field. Electromagnetic field interactions result in linear and/or rotational motion. When only linear motion occurs people talk about an electric field. When only rotational motion occurs people talk about a magnetic field. But these aren't really fields, they're forces. The field concerned is the electromagnetic field. See Jackson’s Classical Electrodynamics on this. See section 11.10 where he says "one should properly speak of the electromagnetic field Fuv rather than E or B separately". Edit: See this re a medium. And do your own research. Don't believe what ignorant people tell you.