A gentle introduction to the foundations of classical electrodynamics:   The meaning of the excitations (D,H) and the field strengths (E, B)
  The axiomatic structure of the electromagnetic theory is outlined. We will base classical electrodynamics on (1) electric charge conservation, (2) the Lorentz force, (3) magnetic flux conservation, and (4) on the Maxwell-Lorentz spacetime relations. This yields the Maxwell equations. The consequences will be drawn, inter alia, for the interpretation and the dimension of the electric and magnetic fields. 
Friedrich W. Hehl, Yuri N. Obukhov